Makefile: Allow one to specify python version (#74517)

Change:
- Allow overriding python version as env var for Makefile
- Including hacking directory in git export, for docs build

Test Plan:
- Tested as part of recent downstream work

Co-authored-by: Yanis Guenane <yguenane@gmail.com>

.gitattributes

@@ -1,2 +1 @@
 .github/ export-ignore
-hacking/ export-ignore

Makefile

@@ -34,7 +34,7 @@ else
 ASCII2MAN = @echo "ERROR: rst2man from docutils command is not installed but is required to build $(MANPAGES)" && exit 1
 endif
 
-PYTHON=python
+PYTHON ?= python
 GENERATE_CLI = hacking/build-ansible.py generate-man
 
 # fetch version from project release.py as single source-of-truth