Use declarative setuptools configuration and PyPI build tools

[mwtoews]

This converts setup.py to setuptools' declarative configuration setup.cfg file. Other changes:

• Use PyPI's build tool in CI to check package build
• Update publish guide to recommend using python -m build and twine check dist/*
• Add empty file xmipy/timers/__init__.py so xmipy.timers becomes a submodule
• setup.py is kept as a shim, but it can be deleted
• Merge (and delete) contents from .flake8 to setup.cfg to clean-up one file

[codecov]

Codecov Report

Merging #73 (61d8269) into develop (ed8d4c2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #73   +/-   ##
========================================
  Coverage    85.87%   85.87%           
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  Files            6        6           
  Lines          439      439           
========================================
  Hits           377      377           
  Misses          62       62           

[Hofer-Julian]

This is strictly better than what we currently have, thanks a lot @mwtoews!

setup.py is kept as a shim, but it can be deleted

Is there a reason to keep it?

[mwtoews]

Some old versions of pip expect a setup.py, but this is probably not an issue since Python 3.7 is the minimum requirement. So sure, let's remove it. To replicate it, use something like:

python -c "from setuptools import setup; setup()" sdist

[pya]

The only reason I am aware of to keep an, essentially empty, setup.py around were editable installs, i.e. pip install --editable. This reason seems to be gone. So, deleting setup.py should be fine.