Check for magic number mismatch early to avoid surprises in RC stage

[ambv]

In RC/final stage this breaks the build. In previous stages it generates a question whether to continue.

This depends on #152. After that lands, change the base branch here to master. Then you can merge the PR.

[ambv]

Wow, that codecov sure is a snitch.

[Yhg1s]

Doh, this broke releases with older versions, which don't have pycore_magic_number.h:

💥  Checking the magic number is up-to-date
Traceback (most recent call last):
  File "./run_release.py", line 1283, in <module>
    main()
  File "./run_release.py", line 1279, in main
    automata.run()
  File "./run_release.py", line 244, in run
    raise e from None
  File "./run_release.py", line 241, in run
    self.current_task(self.db)
  File "release.py", line 126, in __call__
    return getattr(self, "function")(db)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./run_release.py", line 402, in check_magic_number
    magic_actual = get_magic(magic_actual_file, magic_actual_re)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./run_release.py", line 391, in get_magic
    if m := regex.search(source.read_text()):
                         ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/pathlib.py", line 1059, in read_text
    with self.open(mode='r', encoding=encoding, errors=errors) as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/pathlib.py", line 1045, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'cpython/Include/internal/pycore_magic_number.h'