kbuild: remove PYTHON variable

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

As in commit 51839e29 ("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>

Documentation/Makefile

@@ -75,7 +75,7 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
       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 \

Documentation/kbuild/makefiles.rst

@@ -755,7 +755,7 @@ more details, with real examples.
 	bits on the scripts nonetheless.
 
 	Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
-	$(PYTHON) and $(PYTHON3) to refer to interpreters for the respective
+	and $(PYTHON3) to refer to interpreters for the respective
 	scripts.
 
 	Example::

Makefile

@@ -452,7 +452,6 @@ AWK		= awk
 INSTALLKERNEL  := installkernel
 DEPMOD		= depmod
 PERL		= perl
-PYTHON		= python
 PYTHON3		= python3
 CHECK		= sparse
 BASH		= bash
@@ -508,7 +507,7 @@ CLANG_FLAGS :=
 
 export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
-export PERL PYTHON PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
+export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
 export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
 

arch/ia64/Makefile

@@ -69,7 +69,7 @@ vmlinux.bin: vmlinux FORCE
 	$(call if_changed,objcopy)
 
 unwcheck: vmlinux
-	-$(Q)READELF=$(READELF) $(PYTHON) $(srctree)/arch/ia64/scripts/unwcheck.py $<
+	-$(Q)READELF=$(READELF) $(PYTHON3) $(srctree)/arch/ia64/scripts/unwcheck.py $<
 
 archclean:
 

arch/ia64/scripts/unwcheck.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0
 #
 # Usage: unwcheck.py FILE

scripts/jobserver-exec

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0+
 #
 # This determines how many parallel tasks "make" is expecting, as it is