tests: migrate from pytest-cov to coverage

[GraemeWatt]

pytest-cov v4.0.0 drops support for multiprocessing, meaning that test coverage falls from 83% to 72% when upgrading from pytest-cov v3.0.0 to v4.0.0. We should migrate from pytest-cov to coverage as suggested in the pytest-cov Changelog. Until then, we need to keep the pin pytest-cov<4.0.0 introduced in a81d58e4280ccf0b209cea81086d4dbcbdca9003.

Originally posted by @GraemeWatt in https://github.com/HEPData/hepdata/issues/576#issuecomment-1337051695