kbuild: remove PYTHON variable

Python retired in 2020, and some distributions do not provide the
'python' command any more.

As in commit 51839e29cb59 ("scripts: switch explicitly to Python 3"),
we need to use more specific 'python3' to invoke scripts even if they
are written in a way compatible with both Python 2 and 3.

This commit removes the variable 'PYTHON', and switches the existing
users to 'PYTHON3'.

BTW, PEP 394 (https://www.python.org/dev/peps/pep-0394/) is a helpful
material.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 61a7310..9c42dde 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -75,7 +75,7 @@
       cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/userspace-api/media $2 && \
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
-	$(PYTHON) $(srctree)/scripts/jobserver-exec \
+	$(PYTHON3) $(srctree)/scripts/jobserver-exec \
 	$(SHELL) $(srctree)/Documentation/sphinx/parallel-wrapper.sh \
 	$(SPHINXBUILD) \
 	-b $2 \