build: get version with importlib-metadata

[redeboer]

Removes call to pkg_resources.

Closes #80

[codecov-commenter]

Codecov Report

Merging #81 (ae51c09) into master (adaa075) will decrease coverage by 0.32%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   92.75%   92.43%   -0.33%     
==========================================
  Files           8        8              
  Lines         483      489       +6     
  Branches       91       92       +1     
==========================================
+ Hits          448      452       +4     
- Misses         23       24       +1     
- Partials       12       13       +1     

Impacted Files	Coverage Δ
phasespace/__init__.py	90.47% <75.00%> (-9.53%)	:arrow_down:

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[jonas-eschle]

Thanks a lot, LGTM!