[libclang/python] Bump minimum compatibility to Python 3.6 (#77228)

Remove Python 2 support and clean up code that conditions based on
version.

Issue #76664.

Author: linux4life798

clang/bindings/python/README.txt

@@ -10,7 +10,7 @@ runner. For example:
 --
 $ env PYTHONPATH=$(echo ~/llvm/clang/bindings/python/) \
       CLANG_LIBRARY_PATH=$(llvm-config --libdir) \
-  python -m unittest discover -v
+  python3 -m unittest discover -v
 tests.cindex.test_index.test_create ... ok
 ...