gdb/python: remove Python 2 support

New in this version:

 - Add a PY_MAJOR_VERSION check in configure.ac / AC_TRY_LIBPYTHON.  If
   the user passes --with-python=python2, this will cause a configure
   failure saying that GDB only supports Python 3.

Support for Python 2 is a maintenance burden for any patches touching
Python support.  Among others, the differences between Python 2 and 3
string and integer types are subtle.  It requires a lot of effort and
thinking to get something that behaves correctly on both.  And that's if
the author and reviewer of the patch even remember to test with Python
2.

See this thread for an example:

  https://sourceware.org/pipermail/gdb-patches/2021-December/184260.html

So, remove Python 2 support.  Update the documentation to state that GDB
can be built against Python 3 (as opposed to Python 2 or 3).

Update all the spots that use:

 - sys.version_info
 - IS_PY3K
 - PY_MAJOR_VERSION
 - gdb_py_is_py3k

... to only keep the Python 3 portions and drop the use of some
now-removed compatibility macros.

I did not update the configure script more than just removing the
explicit references to Python 2.  We could maybe do more there, like
check the Python version and reject it if that version is not
supported.  Otherwise (with this patch), things will only fail at
compile time, so it won't really be clear to the user that they are
trying to use an unsupported Python version.  But I'm a bit lost in the
configure code that checks for Python, so I kept that for later.

Change-Id: I75b0f79c148afbe3c07ac664cfa9cade052c0c62