[bug#53019] [PATCH 4/6] gnu: Add python-pytest-cov-3.0.

[goodoldpaul]

Dear Maxime,
> Likewise as for python-coverage-5.5, why the separate package?
Both python-pytest-cov and python-coverage have a lot of dependents, 
they must be updated on staging...

$ guix refresh --list-dependent python-pytest-cov
Building the following 521 packages would ensure 1172 dependent packages 
are rebuilt: python-pysox [...]

$ guix refresh --list-dependent python-coverage
Building the following 627 packages would ensure 1408 dependent packages 
are rebuilt: u-boot-puma-rk3399 [...]\

I sent a patch for python-pytest-cov back in march, 
(https://git.savannah.gnu.org/cgit/guix.git/commit/?h=staging&id=12c9da35389dfba86ae0d863132a6b2c4374205a) 
but I didn't manage to find the time to do it also for python-coverage.

Thank you,

Giacomo