tools/docs: python_version: move version check from sphinx-pre-install

tools/docs: python_version: move version check from sphinx-pre-install

The sphinx-pre-install code has some logic to deal with Python
version, which ensures that a minimal version will be enforced
for documentation build logic.

Move it to a separate library to allow re-using its code.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-ID: <d134ace64b55c827565ce68f0527e20c735f0d2e.1758196090.git.mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>