Remove Makefile

[noviluni]

The Makefile is something that hasn't been maintained for a long time.

This package is usually installed through pip, so the make options are not used. On the other hand, for development we currently use tox, which is able to handle:

• testing (pytest) and coverage: tox -e py3
• linting (flake8 through pytest-flake8): tox -e flake8
• docs building: tox -e docs

With this said, I think we don't need the Makefile anymore, and by removing it we can ease the project maintenance.

Closes #892, closes #893

[codecov]

Codecov Report

Merging #907 (d3dd954) into master (21c629f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #907   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files         231      231           
  Lines        2597     2597           
=======================================
  Hits         2552     2552           
  Misses         45       45           

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