Migrate setup.cfg to pyproject.toml

[hugovk]

With help from https://github.com/abravalheri/ini2toml/:

ini2toml setup.cfg -p setup.cfg -o pyproject.toml

But note abravalheri/ini2toml#50.

Flake8 doesn't support pyproject.toml so move its config to .flake8, and delete setup.cfg.

Also still needed setup_requires=["setuptools_scm"], in setup.py, because otherwise Python 3.7-3.9 (but not 3.10-3.11) failed on the CI:

error: error in 'egg_base' option: 'src' does not exist or is not a directory

Details

================================== log start ===================================
Processing ./.tox/.tmp/package/1/UNKNOWN-0.0.0.zip
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'error'
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [4 lines of output]
      running egg_info
      /tmp/pip-build-env-jofklcr1/overlay/lib/python3.7/site-packages/setuptools/config/pyprojecttoml.py:108: _BetaConfiguration: Support for `[tool.setuptools]` in `pyproject.toml` is still *beta*.
        warnings.warn(msg, _BetaConfiguration)
      error: error in 'egg_base' option: 'src' does not exist or is not a directory
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

=================================== log end ====================================

And https://github.com/pypa/setuptools_scm#pyprojecttoml-usage says:

Many tools, especially those that invoke setup.py for any reason, may
continue to rely on setup_requires. For maximum compatibility
with those uses, consider also including a setup_requires directive
(described below in setup.py usage and setup.cfg).

[codecov]

Codecov Report

Merging #354 (412f907) into main (a89adec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files           8        8           
  Lines         777      777           
=======================================
  Hits          773      773           
  Misses          4        4           

Flag	Coverage Δ
macos-latest	99.48% <ø> (ø)
ubuntu-latest	99.48% <ø> (ø)
windows-latest	99.48% <ø> (ø)

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