[Proposal] Add coverage

[cvanelteren]

We should add pyest-cov to the package to improve info on what we could improve upon with regards to tests

Currently, we hit 79 percent

---------- coverage: platform linux, python 3.11.0-final-0 -----------
Name                                           Stmts   Miss  Cover
------------------------------------------------------------------
ultraplot/__init__.py                             81     10    88%
ultraplot/axes/__init__.py                        18      0   100%
ultraplot/axes/base.py                          1343    225    83%
ultraplot/axes/cartesian.py                      485     68    86%
ultraplot/axes/geo.py                            749    138    82%
ultraplot/axes/plot.py                          1880    322    83%
ultraplot/axes/polar.py                           91      6    93%
ultraplot/axes/shared.py                         124      9    93%
ultraplot/axes/three.py                           12      2    83%
ultraplot/colors.py                             1172    367    69%
ultraplot/config.py                              755    216    71%
ultraplot/constructor.py                         424    106    75%
ultraplot/demos.py                               315    280    11%
ultraplot/externals/__init__.py                    1      0   100%
ultraplot/externals/hsluv.py                     210     45    79%
ultraplot/figure.py                              817    158    81%
ultraplot/gridspec.py                            869    142    84%
ultraplot/internals/__init__.py                  182     23    87%
ultraplot/internals/benchmarks.py                 12      2    83%
ultraplot/internals/context.py                    21      0   100%
ultraplot/internals/docstring.py                  48      3    94%
ultraplot/internals/fonts.py                      37     24    35%
ultraplot/internals/guides.py                     78     15    81%
ultraplot/internals/inputs.py                    482    113    77%
ultraplot/internals/labels.py                     44      2    95%
ultraplot/internals/rcsetup.py                   360    113    69%
ultraplot/internals/versions.py                   35     10    71%
ultraplot/internals/warnings.py                   66     14    79%
ultraplot/proj.py                                105     32    70%
ultraplot/scale.py                               398    148    63%
ultraplot/tests/__init__.py                       16      9    44%
ultraplot/tests/test_1dplots.py                  240      2    99%
ultraplot/tests/test_2dplots.py                  201     12    94%
ultraplot/tests/test_axes.py                      96      0   100%
ultraplot/tests/test_colorbar.py                 119      7    94%
ultraplot/tests/test_constructor.py               56     11    80%
ultraplot/tests/test_docs.py                       0      0   100%
ultraplot/tests/test_format.py                   146      2    99%
ultraplot/tests/test_geographic.py                59      0   100%
ultraplot/tests/test_imshow.py                    58      2    97%
ultraplot/tests/test_inset.py                     10      0   100%
ultraplot/tests/test_integration.py               62      0   100%
ultraplot/tests/test_legend.py                   103      0   100%
ultraplot/tests/test_projections.py               66      5    92%
ultraplot/tests/test_statistical_plotting.py      43      0   100%
ultraplot/tests/test_subplots.py                 120      0   100%
ultraplot/ticker.py                              382    117    69%
ultraplot/ui.py                                   69      6    91%
ultraplot/utils.py                               255     72    72%
------------------------------------------------------------------
TOTAL                                          13315   2838    79%

[cvanelteren]

Actually without the tests dir its 77 ;-)

[cvanelteren]

Merge in main #121 .