TST: improve coverage

[mdhaber]

Closes gh-32
After this and gh-34, coverage (according to coverage) should be about as high as possible.

[mdhaber]

OK, I think I'll call this good.

---------- coverage: platform linux, python 3.10.15-final-0 ----------
Name                          Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------------
marray/__init__.py              326      6     48      0    98%   414-420
marray/tests/test_marray.py     630      3     96      1    99%   72-73, 95
-------------------------------------------------------------------------
TOTAL                           956      9    144      1    99%

414-420 is the set functions (gh-34) and 72-73/95 are for when assertions fail (they re-raise with the seed to reproduce the problem).

[mdhaber]

@keewis is there any way for a CI to fail if coverage changes for the worse?

[keewis]

multiple ways, even: there's codecov/codecov-action (add --cov=marray --cov-report=xml to the pytest command and install pytest-cov), or you can follow the setup described at https://hynek.me/articles/ditch-codecov-python

I personally print the coverage report in CI and check manually, but it's easy forget to do when reviewing (on the other hand, coverage is a metric that we shouldn't really follow religiously)