docs: changes: better document Python needs

Python is listed as an optional dependency, but this is not
true, as:

1) arm (multi_v7_defconfig and other defconfigs) and arm64 defconfig
needs it due to DRM_MSM dependencies;

2) CONFIG_LTO_CLANG runs a python script at scripts/Makefile.vmlinux_o;

3) kernel-doc is called during compilation when some DRM options
like CONFIG_DRM_I915_WERROR are enabled;

4) allyesconfig/allmodconfig will enable CONFIG_* dependencies
that needs it;

5) besides DRM, other subsystems seem to have logic calling *.py
scripts.

So, better document that and change the dependency from optional
to mandatory to reflect the current needs.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Acked-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/b03b95[e-mail]

Documentation/process/changes.rst

···
 GNU tar                1.28             tar --version
 gtags (optional)       6.6.5            gtags --version
 mkimage (optional)     2017.01          mkimage --version
-Python (optional)      3.9.x            python3 --version
+Python                 3.9.x            python3 --version
 GNU AWK (optional)     5.1.0            gawk --version
 ====================== ===============  ========================================
 
···
 
 You will need perl 5 and the following modules: ``Getopt::Long``,
 ``Getopt::Std``, ``File::Basename``, and ``File::Find`` to build the kernel.
+
+Python
+------
+
+Several config options require it: it is required for arm/arm64
+default configs, CONFIG_LTO_CLANG, some DRM optional configs,
+the kernel-doc tool, and docs build (Sphinx), among others.
 
 BC
 --