fix(enum): replace read-only enum values

Some enum values like `MSO_SHAPE_TYPE.MIXED` are never produced by
`python-pptx` and so were removed during the enum modernization in
commit `01b86e64`. However, in at least one downstream system these
values are referenced even though they can never actually occur.

Replace these "read-only" values to avoid downstream breakage.

src/pptx/enum/chart.py

@@ -335,6 +335,9 @@ class XL_DATA_LABEL_POSITION(BaseXmlEnum):
     LEFT = (-4131, "l", "The data label is positioned to the left of the data point.")
     """The data label is positioned to the left of the data point."""
 
+    MIXED = (6, "", "Data labels are in multiple positions (read-only).")
+    """Data labels are in multiple positions (read-only)."""
+
     OUTSIDE_END = (
         2,
         "outEnd",
@@ -370,8 +373,8 @@ class XL_LEGEND_POSITION(BaseXmlEnum):
     CORNER = (2, "tr", "In the upper-right corner of the chart border.")
     """In the upper-right corner of the chart border."""
 
-    CUSTOM = (-4161, "", "A custom position.")
-    """A custom position."""
+    CUSTOM = (-4161, "", "A custom position (read-only).")
+    """A custom position (read-only)."""
 
     LEFT = (-4131, "l", "Left of the chart.")
     """Left of the chart."""

src/pptx/enum/dml.py

@@ -319,8 +319,8 @@ class MSO_PATTERN_TYPE(BaseXmlEnum):
     ZIG_ZAG = (38, "zigZag", "Zig Zag")
     """Zig Zag"""
 
-    MIXED = (-2, "", "Mixed pattern")
-    """Mixed pattern"""
+    MIXED = (-2, "", "Mixed pattern (read-only).")
+    """Mixed pattern (read-only)."""
 
 
 MSO_PATTERN = MSO_PATTERN_TYPE
@@ -394,8 +394,12 @@ class MSO_THEME_COLOR_INDEX(BaseXmlEnum):
     TEXT_2 = (15, "tx2", "Specifies the Text 2 theme color.")
     """Specifies the Text 2 theme color."""
 
-    MIXED = (-2, "", "Indicates multiple theme colors are used, such as in a group shape.")
-    """Indicates multiple theme colors are used, such as in a group shape."""
+    MIXED = (
+        -2,
+        "",
+        "Indicates multiple theme colors are used, such as in a group shape (read-only).",
+    )
+    """Indicates multiple theme colors are used, such as in a group shape (read-only)."""
 
 
 MSO_THEME_COLOR = MSO_THEME_COLOR_INDEX

src/pptx/enum/lang.py

@@ -680,3 +680,6 @@ class MSO_LANGUAGE_ID(BaseXmlEnum):
 
     ZULU = (1077, "zu-ZA", "The Zulu language.")
     """The Zulu language."""
+
+    MIXED = (-2, "", "More than one language in specified range (read-only).")
+    """More than one language in specified range (read-only)."""

src/pptx/enum/shapes.py

@@ -748,6 +748,9 @@ class MSO_CONNECTOR_TYPE(BaseXmlEnum):
     STRAIGHT = (1, "line", "Straight line connector.")
     """Straight line connector."""
 
+    MIXED = (-2, "", "Return value only; indicates a combination of other states.")
+    """Return value only; indicates a combination of other states."""
+
 
 MSO_CONNECTOR = MSO_CONNECTOR_TYPE
 
@@ -843,6 +846,9 @@ class MSO_SHAPE_TYPE(BaseEnum):
     WEB_VIDEO = (26, "Web video")
     """Web video"""
 
+    MIXED = (-2, "Multiple shape types (read-only).")
+    """Multiple shape types (read-only)."""
+
 
 MSO = MSO_SHAPE_TYPE
 
@@ -871,6 +877,16 @@ class PP_MEDIA_TYPE(BaseEnum):
     SOUND = (1, "Audio media such as MP3.")
     """Audio media such as MP3."""
 
+    MIXED = (
+        -2,
+        "Return value only; indicates multiple media types, typically for a collection of shapes."
+        " May not be applicable in python-pptx.",
+    )
+    """Return value only; indicates multiple media types.
+
+    Typically for a collection of shapes. May not be applicable in python-pptx.
+    """
+
 
 class PP_PLACEHOLDER_TYPE(BaseXmlEnum):
     """Specifies one of the 18 distinct types of placeholder.
@@ -937,14 +953,14 @@ class PP_PLACEHOLDER_TYPE(BaseXmlEnum):
     TITLE = (1, "title", "Title")
     """Title"""
 
-    VERTICAL_BODY = (6, "", "Vertical Body")
-    """Vertical Body"""
+    VERTICAL_BODY = (6, "", "Vertical Body (read-only).")
+    """Vertical Body (read-only)."""
 
-    VERTICAL_OBJECT = (17, "", "Vertical Object")
-    """Vertical Object"""
+    VERTICAL_OBJECT = (17, "", "Vertical Object (read-only).")
+    """Vertical Object (read-only)."""
 
-    VERTICAL_TITLE = (5, "", "Vertical Title")
-    """Vertical Title"""
+    VERTICAL_TITLE = (5, "", "Vertical Title (read-only).")
+    """Vertical Title (read-only)."""
 
     MIXED = (-2, "", "Return value only; multiple placeholders of differing types.")
     """Return value only; multiple placeholders of differing types."""

src/pptx/enum/text.py

@@ -57,6 +57,9 @@ class MSO_AUTO_SIZE(BaseEnum):
     )
     """The font size is reduced as necessary to fit the text within the shape."""
 
+    MIXED = (-2, "Return value only; indicates a combination of automatic sizing schemes are used.")
+    """Return value only; indicates a combination of automatic sizing schemes are used."""
+
 
 class MSO_TEXT_UNDERLINE_TYPE(BaseXmlEnum):
     """
@@ -134,6 +137,9 @@ class MSO_TEXT_UNDERLINE_TYPE(BaseXmlEnum):
     WORDS = (1, "words", "Specifies underlining words.")
     """Specifies underlining words."""
 
+    MIXED = (-2, "", "Specifies a mix of underline types (read-only).")
+    """Specifies a mix of underline types (read-only)."""
+
 
 MSO_UNDERLINE = MSO_TEXT_UNDERLINE_TYPE
 
@@ -161,6 +167,9 @@ class MSO_VERTICAL_ANCHOR(BaseXmlEnum):
     BOTTOM = (4, "b", "Aligns text to bottom of text frame")
     """Aligns text to bottom of text frame"""
 
+    MIXED = (-2, "", "Return value only; indicates a combination of the other states.")
+    """Return value only; indicates a combination of the other states."""
+
 
 MSO_ANCHOR = MSO_VERTICAL_ANCHOR
 
@@ -214,5 +223,8 @@ class PP_PARAGRAPH_ALIGNMENT(BaseXmlEnum):
     THAI_DISTRIBUTE = (6, "thaiDist", "Thai distributed")
     """Thai distributed"""
 
+    MIXED = (-2, "", "Multiple alignments are present in a set of paragraphs (read-only).")
+    """Multiple alignments are present in a set of paragraphs (read-only)."""
+
 
 PP_ALIGN = PP_PARAGRAPH_ALIGNMENT