Reduce min Python version from 3.8 to 3.6 (#610)

The default version of `python3` on RHEL8 machines in Python 3.6.8.  (I am not
sure where I got Python 3.8 from.)

But note that RHEL8 machines also seem to have Python 3.11 and 3.12 installed
as well.  I am wondering if it would not be more wise to be pro-active and
push the version of Python being required to a higher version that will be
supported for longer.  (Python 3.6 ended support at the end of 2021.)

test/core/TestingFunctionMacro_UnitTests.cmake

@@ -195,23 +195,23 @@ function(tribits_find_python_interp_uses_python_empty)
   message("tribits_find_python_interp(): ${PROJECT_NAME}_USES_PYTHON=''")
   global_set(MESSAGE_WRAPPER_INPUT "")
   set(${PROJECT_NAME}_USES_PYTHON "")
-  set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.8)
+  set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.6)
   tribits_find_python_interp()
   unittest_compare_const(MESSAGE_WRAPPER_INPUT
-    "-- ;Python3_EXECUTABLE='/path/to/python3.8'")
+    "-- ;Python3_EXECUTABLE='/path/to/python3.6'")
 endfunction()
 
 
 function(tribits_find_python_interp_uses_python_true)
   message("tribits_find_python_interp(): ${PROJECT_NAME}_USES_PYTHON=TRUE")
   global_set(MESSAGE_WRAPPER_INPUT)
   set(${PROJECT_NAME}_USES_PYTHON TRUE)
-  global_set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.8)
+  global_set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.6)
   tribits_find_python_interp()
   unittest_compare_const(MESSAGE_WRAPPER_INPUT
-    "-- ;Python3_EXECUTABLE='/path/to/python3.8'")
+    "-- ;Python3_EXECUTABLE='/path/to/python3.6'")
   unittest_compare_const(FIND_Python3_ARGS
-    "Python3;3.8")
+    "Python3;3.6")
 endfunction()
 
 
@@ -220,23 +220,23 @@ function(tribits_find_python_interp_requires_python_true)
   global_set(MESSAGE_WRAPPER_INPUT)
   set(${PROJECT_NAME}_USES_PYTHON FALSE)
   set(${PROJECT_NAME}_REQUIRES_PYTHON TRUE)
-  set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.8)
+  set(Python3_EXECUTABLE_UNITTEST_VAL /path/to/python3.6)
   tribits_find_python_interp()
   unittest_compare_const(MESSAGE_WRAPPER_INPUT
-    "-- ;Python3_EXECUTABLE='/path/to/python3.8'")
+    "-- ;Python3_EXECUTABLE='/path/to/python3.6'")
   unittest_compare_const(FIND_Python3_ARGS
-    "Python3;3.8;REQUIRED")
+    "Python3;3.6;REQUIRED")
 endfunction()
 
 
 function(tribits_find_python_interp_find_version_too_low)
-  message("tribits_find_python_interp(): ${PROJECT_NAME}_Python3_FIND_VERSION=3.7 (too low)")
+  message("tribits_find_python_interp(): ${PROJECT_NAME}_Python3_FIND_VERSION=3.5 (too low)")
   global_set(MESSAGE_WRAPPER_INPUT)
-  set(${PROJECT_NAME}_Python3_FIND_VERSION 3.7)
+  set(${PROJECT_NAME}_Python3_FIND_VERSION 3.5)
   set(Python3_EXECUTABLE_UNITTEST_VAL /dummy)
   tribits_find_python_interp()
   unittest_compare_const(MESSAGE_WRAPPER_INPUT
-    "FATAL_ERROR;Error,; ${PROJECT_NAME}_Python3_FIND_VERSION=3.7 < 3.8; is not allowed!;-- ;Python3_EXECUTABLE='/dummy'")
+    "FATAL_ERROR;Error,; ${PROJECT_NAME}_Python3_FIND_VERSION=3.5 < 3.6; is not allowed!;-- ;Python3_EXECUTABLE='/dummy'")
 endfunction()
 
 
@@ -262,7 +262,7 @@ function(tribits_find_python_interp_python_executable_backward_compatible)
   unittest_compare_const(MESSAGE_WRAPPER_INPUT
     "DEPRECATION;Python3_EXECUTABLE being set by default to PYTHON_EXECUTABLE = '/path/to/python3.10.2' is deprecated!;\n\nNOTE: To Make these warnings go away, set -D; TRIBITS_HANDLE_TRIBITS_DEPRECATED_CODE=IGNORE (see the build reference guide).;-- ;Python3_EXECUTABLE='/path/to/python3.10.2'")
   unittest_compare_const(FIND_Python3_ARGS
-    "Python3;3.8")
+    "Python3;3.6")
 endfunction()
 
 

tribits/core/package_arch/TribitsFindPythonInterp.cmake

@@ -42,7 +42,7 @@ endmacro()
 
 macro(tribits_find_python_set_python3_find_version)
   # Get minimum version of Python to find
-  set(${PROJECT_NAME}_Python3_FIND_VERSION_MIN "3.8")
+  set(${PROJECT_NAME}_Python3_FIND_VERSION_MIN "3.6")
   if ("${${PROJECT_NAME}_Python3_FIND_VERSION_DEFAULT}" STREQUAL "")
     set(${PROJECT_NAME}_Python3_FIND_VERSION_DEFAULT
       "${${PROJECT_NAME}_Python3_FIND_VERSION_MIN}")

tribits/doc/guides/TribitsGuidesBody.rst

@@ -7003,9 +7003,9 @@ Python Support
 
 TriBITS Core does not require anything other than raw CMake.  However, Python
 Utils, TriBITS CI Support, and other extended TriBITS components require
-Python.  These extra TriBITS tools only require Python 3.8+.  By default, when
+Python.  These extra TriBITS tools only require Python 3.6+.  By default, when
 a TriBITS project starts to configure using CMake, it will try to find Python
-3.8+ on the system (see `Full Processing of TriBITS Project Files`_).  If
+3.6+ on the system (see `Full Processing of TriBITS Project Files`_).  If
 Python is found, it will set the global cache variable ``Python3_EXECUTABLE``.
 If it is not found, then it will print a warning and ``Python3_EXECUTABLE``
 will be empty.  With this default behavior, if Python is found, then the
@@ -7022,9 +7022,9 @@ many TriBITS projects, some TriBITS projects need different behavior such as:
    can't be found.  In this case, the TriBITS project would set
    `${PROJECT_NAME}_REQUIRES_PYTHON`_ to ``TRUE``.
 
-3. Some TriBITS projects may require a version of Python more recent than 3.8.
+3. Some TriBITS projects may require a version of Python more recent than 3.6.
    In this case, the TriBITS project would set
-   `${PROJECT_NAME}_Python3_FIND_VERSION`_ to some value higher than ``3.8``.
+   `${PROJECT_NAME}_Python3_FIND_VERSION`_ to some value higher than ``3.6``.
    For example, may newer systems have higher versions of Python installed by
    default, and projects developed on such a system typically require this
    version or higher.