Add some undocumented constants to gzip.pyi (#8675)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Alex Waygood <[email protected]>

stdlib/gzip.pyi

@@ -15,8 +15,14 @@ _ReadBinaryMode: TypeAlias = Literal["r", "rb"]
 _WriteBinaryMode: TypeAlias = Literal["a", "ab", "w", "wb", "x", "xb"]
 _OpenTextMode: TypeAlias = Literal["rt", "at", "wt", "xt"]
 
-READ: Literal[1]
-WRITE: Literal[2]
+READ: Literal[1]  # undocumented
+WRITE: Literal[2]  # undocumented
+
+FTEXT: int  # actually Literal[1] # undocumented
+FHCRC: int  # actually Literal[2] # undocumented
+FEXTRA: int  # actually Literal[4] # undocumented
+FNAME: int  # actually Literal[8] # undocumented
+FCOMMENT: int  # actually Literal[16] # undocumented
 
 class _ReadableFileobj(Protocol):
     def read(self, __n: int) -> bytes: ...