FindPython: Add support for version 3.9

Development versions of Python 3.9.0 are already out there.

See PEP 596 -- Python 3.9 Release Schedule:
https://www.python.org/dev/peps/pep-0596/

Modules/FindPython/Support.cmake

@@ -23,7 +23,7 @@ if (NOT DEFINED _${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR)
   message (FATAL_ERROR "FindPython: INTERNAL ERROR")
 endif()
 if (_${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR EQUAL 3)
-  set(_${_PYTHON_PREFIX}_VERSIONS 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
+  set(_${_PYTHON_PREFIX}_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
 elseif (_${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR EQUAL 2)
   set(_${_PYTHON_PREFIX}_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0)
 else()

Modules/FindPythonInterp.cmake

@@ -54,7 +54,7 @@ unset(_Python_NAMES)
 
 set(_PYTHON1_VERSIONS 1.6 1.5)
 set(_PYTHON2_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0)
-set(_PYTHON3_VERSIONS 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
+set(_PYTHON3_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
 
 if(PythonInterp_FIND_VERSION)
     if(PythonInterp_FIND_VERSION_COUNT GREATER 1)

Modules/FindPythonLibs.cmake

@@ -79,7 +79,7 @@ set(CMAKE_FIND_FRAMEWORK LAST)
 
 set(_PYTHON1_VERSIONS 1.6 1.5)
 set(_PYTHON2_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0)
-set(_PYTHON3_VERSIONS 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
+set(_PYTHON3_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
 
 if(PythonLibs_FIND_VERSION)
     if(PythonLibs_FIND_VERSION_COUNT GREATER 1)