Use Python "Limited API" in PL/Python

This patch changes PL/Python to use the Python "limited API". This API 
has stronger ABI stability guarantees.[0] This means, you can build 
PL/Python against any Python 3.x version and use any other Python 3.x 
version at run time.

This is especially useful for binary packages where the operating system 
does not come with a fixed suitable version of Python. For example, 
Postgres.app (for macOS) would prefer to link against the Python version 
supplied by python.org (Python.app). But that has a 3.x version that 
changes over time. So instead they bundle a Python version inside 
Postgres.app. The Windows installer used to also bundle Python but as of 
PG17 you have to get it yourself, but you have to get a very specific 
version [1], which is unsatisfactory. This patch fixes that: You can use 
any Python version independent of what PL/Python was built against. 
(There is a mechanism to say "at least 3.N", but for this patch, we 
don't need that, we can stick with the current minimum of 3.2.)

(I have only tested the macOS side of this, not the Windows side. In 
fact, the patch currently doesn't build on Windows on CI. I haven't 
figured out why.)

For Linux-style packaging, I don't think this would have any benefit for 
users right now, since the OS comes with a Python installation and all 
the packages are built against that. But it could potentially be helpful 
for packagers. For example, on Debian, this could detach the postgresql 
packages from python version transitions. But AFAICT, the Python 
packaging layout is not prepared for that. (There are only 
libpython3.x.so libraries, no libpython3.so that one would have to link 
against.)

Finally, I think this patch is part of a path toward making PL/Python 
thread-safe. I don't think the patch by itself changes anything, but if 
you read through [2], using heap types is part of the things mentioned 
there.

[0]: https://docs.python.org/3/c-api/stable.html
[1]: 
https://github.com/EnterpriseDB/edb-installers/blob/REL-17/server/resources/installation-notes.html#L34-L36
[2]: https://docs.python.org/3/howto/isolating-extensions.html