chore: Use newer setuptools_scm for versioning

[matthewfeickert]

This bumps us up to a more modern version of setuptools_scm that supports pyproject.toml and will inject the right version at build time (via PEP517/8) as well as generate the corresponding version.py for us at build time.

c.f. scikit-hep/pyhf#1450 for motivation

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• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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* Update to a modern version of setuptools_scm to support pyproject-based Python packages
   - Use local_scheme = "no-local-version" to continue uploading to TestPyPI
   - c.f. https://github.com/pypa/setuptools_scm/blob/v6.0.1/README.rst#version-number-construction
* Add `pylhe.__version__` to API
* Generate src/pylhe/_version.py at build time
   - Remove setup.cfg from bump2version control
* Set fetch depth:0 in CI to get full history and tags to generate version number
   - c.f. https://github.com/scikit-hep/pyhf/issues/1465

[codecov]

Codecov Report

Merging #94 (2fb67ac) into master (34c4b8c) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   60.69%   60.89%   +0.19%     
==========================================
  Files           2        2              
  Lines         201      202       +1     
  Branches       36       36              
==========================================
+ Hits          122      123       +1     
  Misses         72       72              
  Partials        7        7              

Flag	Coverage Δ
unittests	60.89% <100.00%> (+0.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pylhe/__init__.py	66.66% <100.00%> (+0.20%)	⬆️

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