Update Python version (#9230)

Author: hugovk

winbuild/README.md

@@ -16,7 +16,7 @@ For more extensive info, see the [Windows build instructions](build.rst).
 Here's an example script to build on Windows:
 
 ```
-set PYTHON=C:\Python39\bin
+set PYTHON=C:\Python310\bin
 cd /D C:\Pillow\winbuild
 %PYTHON%\python.exe build_prepare.py -v --depends=C:\pillow-depends
 build\build_dep_all.cmd

winbuild/build.rst

@@ -115,7 +115,7 @@ Example
 
 Here's an example script to build on Windows::
 
-    set PYTHON=C:\Python39\bin
+    set PYTHON=C:\Python310\bin
     cd /D C:\Pillow\winbuild
     %PYTHON%\python.exe build_prepare.py -v --depends C:\pillow-depends
     build\build_dep_all.cmd