Clean up tool configurations

[plannigan]

• Change black's target python version
• Switch black to extend-exclude. Most (all?) of the entries are excluded by black by default. it is better to only list the things that are specific to the project.
• Change isort to use black profile. This will simplify the number of settings that need to be specified because isort comes with a profile that matches how black formats code.
• Use testpaths in pytest instead the python_files setting that only matches files in the tests directory.

Closes #275

[codecov]

Codecov Report

Merging #291 (3bb2ecc) into main (a0d5cb7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #291   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files           6        6           
  Lines         347      347           
  Branches       51       51           
=======================================
  Hits          336      336           
  Misses          9        9           
  Partials        2        2           

[plannigan]

You should include all Python versions that you want your code to run under. In practice, though, 3.6 through 3.8 are currently the same as far as Black is concerned ...

(Source: psf/black#751)

[joaufi]

LGTM- should see a decent speedup by using testpaths instead of python_files as the test suite grows.