feat: Use module level __dir__ to restrict public API views

[matthewfeickert]

Description

Implemented from Brett Cannon's blog An approach to lazy importing in Python 3.7 and implemented it for hist in scikit-hep/hist#179:

I’ve used Python 3.7 module-level __getattr__ for a while now to allow mistyping hist.axis import as hist.axes to show a warning but work anyway. However, I completely missed a fantastic usage for Python 3.7 module-level __dir__ : you can “fix” IPython’s tab completion to just show the items in __all__! Wow! One of the main things that bugged me about importing and defining unrelated things in modules, and one reason that boost-histogram has an _internal module.

c.f. scikit-hep/pyhf#1402

Used in PR #83

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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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For the PR Assignees:

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* Use module level __dir__ in combination with __all__ to restrict the public API import views to useful imports
   - Only available in Python 3.7+
* c.f. https://snarky.ca/lazy-importing-in-python-3-7/
* Add tests for output of dir(pylhe)

[codecov]

Codecov Report

Merging #84 (eb2f579) into master (a084774) will increase coverage by 0.63%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   65.62%   66.25%   +0.63%     
==========================================
  Files           1        1              
  Lines         160      163       +3     
  Branches       30       30              
==========================================
+ Hits          105      108       +3     
  Misses         48       48              
  Partials        7        7              

Flag	Coverage Δ
unittests	66.25% <100.00%> (+0.63%)	⬆️

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Impacted Files	Coverage Δ
src/pylhe/__init__.py	66.25% <100.00%> (+0.63%)	⬆️

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