python310 and compiler requirements

Revision 1.176 to pyversion.mk by adam on 08-Oct-25 dropped python310
from PYTHON_VERSION_ACCEPTED. I missed this.

I'm still relying on python310, and will be for the forseeable future.
I'm also shipping python310 to users. Popular software like yt-dlp
works absolutely fine and will for a few more years at least.

It's the last version to build without a C11 compiler. In pkgsrc, this
is taken to mean every gcc version after 4.8. This excludes all GPLv2
versions of GCC, including all versions of GCC supported by UnixWare 5 
(which has active users) and OS X 10.4 (which has active users). It
also excludes NetBSD 7.

Since developers keep changing packages to build with meson without
much thought to compatibility with non-NetBSD platforms, python is
essential to use pkgsrc.

Either keep python310, or raise pkgsrc's bootstrap requirement to C11
and stop pretending. Thanks.