Also run tests without extra dependencies

[Gallaecio]

Continuation of #22 which also prevents extra packages from being imported during test execution even if they are installed.

Fixes #10, closes #22

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (28c1f6e) to head (8ea7432).
⚠️ Report is 123 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #33   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          197       197           
=========================================
  Hits           197       197           

Flag	Coverage Δ
tests	100.00% <ø> (?)

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