use openbabel native pbc implementation

[njzjz]

Removes scipy from dependencies.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ac52ca6). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head f0bbf95 differs from pull request most recent head c24cc2e. Consider uploading reports for the commit c24cc2e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             master     #1744   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?         1           
  Lines             ?        16           
  Branches          ?         0           
==========================================
  Hits              ?        16           
  Misses            ?         0           
  Partials          ?         0           

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