Drop support for Python 3.7?

[Stefan Weil]

Hi,

I just saw that the documentation still mentions that QEMU supports 
Python 3.7.

Python 3.7 is an unsupported Python version since about one year. 
Therefore I suggest to update the documentation for QEMU 9.1.0 and 
replace 3.7 by 3.8 as lowest supported version.

In addition the code which still mentions Python 3.7 or even 3.5 and 3.6 
could be reviewed and maybe simplified, but I think this is less urgent 
and can be done after QEMU release 9.1.0.

Regards
Stefan W.

$ git grep -i python.*3.7
docs/about/build-platforms.rst:  As of QEMU |version|, the minimum 
supported version of Python is 3.7.
python/qemu/qmp/util.py:Python 3.7+.
scripts/qapi/introspect.py:    # TODO: Remove after Python 3.7 adds 
@dataclass:
scripts/qapi/source.py:    # Replace with @dataclass in Python 3.7+
tests/qapi-schema/test-qapi.py:        # dict (requires Python 3.7)