Update python3 framework detection to use 3.9

MacVim's CI build script was already updated to use Python 3.9 from
Homebrew when building, so it would use that automatically, but update
the auto-detection used for MacPorts and binary install support to use
3.9 as well.

Author: ychin

src/MacVim/vimrc

@@ -32,12 +32,12 @@ endif
 " or an installation from python.org:
 if exists("&pythonthreedll") && exists("&pythonthreehome") &&
       \ !filereadable(&pythonthreedll)
-  if filereadable("/opt/local/Library/Frameworks/Python.framework/Versions/3.8/Python")
-    " MacPorts python 3.8
-    set pythonthreedll=/opt/local/Library/Frameworks/Python.framework/Versions/3.8/Python
-  elseif filereadable("/Library/Frameworks/Python.framework/Versions/3.8/Python")
+  if filereadable("/opt/local/Library/Frameworks/Python.framework/Versions/3.9/Python")
+    " MacPorts python 3.9
+    set pythonthreedll=/opt/local/Library/Frameworks/Python.framework/Versions/3.9/Python
+  elseif filereadable("/Library/Frameworks/Python.framework/Versions/3.9/Python")
     " https://www.python.org/downloads/mac-osx/
-    set pythonthreedll=/Library/Frameworks/Python.framework/Versions/3.8/Python
+    set pythonthreedll=/Library/Frameworks/Python.framework/Versions/3.9/Python
   endif
 endif