Enabling pytest code coverage output by default

[jnke2016]

added the coverage plugin option by default
added pytest-cov package

[codecov-io]

Codecov Report

Merging #1352 (acca78e) into branch-0.18 (2fb0725) will decrease coverage by 1.42%.
The diff coverage is 57.84%.

@@               Coverage Diff               @@
##           branch-0.18    #1352      +/-   ##
===============================================
- Coverage        60.38%   58.95%   -1.43%     
===============================================
  Files               67       67              
  Lines             3029     3060      +31     
===============================================
- Hits              1829     1804      -25     
- Misses            1200     1256      +56     

Impacted Files	Coverage Δ
python/cugraph/centrality/__init__.py	100.00% <ø> (ø)
python/cugraph/comms/comms.py	34.52% <25.00%> (ø)
python/cugraph/dask/common/input_utils.py	23.07% <28.57%> (+1.14%)	⬆️
python/cugraph/utilities/utils.py	42.96% <35.71%> (-28.59%)	⬇️
python/cugraph/dask/common/mg_utils.py	37.50% <38.09%> (-2.50%)	⬇️
python/cugraph/community/spectral_clustering.py	72.54% <38.46%> (-11.67%)	⬇️
python/cugraph/structure/number_map.py	58.12% <50.00%> (+2.16%)	⬆️
python/cugraph/structure/graph.py	68.75% <76.47%> (+1.95%)	⬆️
python/cugraph/__init__.py	100.00% <100.00%> (ø)
...ython/cugraph/centrality/betweenness_centrality.py	100.00% <100.00%> (ø)
... and 11 more

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[BradReesWork]

@gpucibot merge

[BradReesWork]

rerun tests

[rlratzel]

rerun tests

[BradReesWork]

@gpucibot merge