pyproject.toml: Remove dependency mock

[cclauss]

#153 makes https://pypi.org/project/mock unnecessary.

mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards.

[codecov-commenter]

Codecov Report

Merging #156 (f3953e8) into master (7d8b252) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #156   +/-   ##
=======================================
  Coverage   97.27%   97.27%           
=======================================
  Files          23       23           
  Lines        1213     1213           
=======================================
  Hits         1180     1180           
  Misses         33       33           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7d8b252...f3953e8. Read the comment docs.

[tonybaloney]

thanks!