[LyX/ next] Prepare the removal of Python 2 support

[José Matos]

In order to pay the technical debt that is the Python 2 support I
intend to start removing its support.

The first step is to change the build support to ignore Python 2.

Since that is code that, usually, I run away from I would like to get
your feedback before committing it. Because if this breaks the package
does not build and that is a huge problem. :-)

@Jean-Marc is the code OK for the autotools?
@Kornel is the code right for cmake?


There is another change that deserves to be discussed, in the process I
have bumped the minimum python version to be 3.10.

Assuming that the next development cycle takes 2.5 years this will be a
version that will be 5 years old by then and so it should be widely
available.


In this particular case the reason to jump directly to 3.10 is the
structural pattern matching:

https://docs.python.org/3/whatsnew/3.10.html#pep-634-structural-pattern-matching

FWIW notice that this is a lot more powerful than the usual C/C++
switch.
I think that some of our code, like lyx2lyx, can take direct advantage
of this feature.

Best regards,
-- 
José Abílio
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