Improve coverage collection

[tobiasdiez]

The current coverage collection is relatively basic as one only keeps track whether a function has a (doc)test or not. On the other hand, there are more advanced tools like coverage.py that give more detailed information about which lines and branches are actually executed.
These reports can uploaded e.g. to codecov to get a nice visual representation of the coverage and track changes in coverage (in particular for PRs).

In this ticket, we integrate coverage.py in the sage workflow. In particular,

1. Add configuration for coverage.py.
2. Add tox target to run it.
3. Add github workflow that uploads coverage to codecov. (maybe as a follow-up ticket)

This is part of #28936 and will improve integration of pytest with the coverage collection.

Depends on #33355

CC: @mkoeppe @tscrim @mjungmath @egourgoulhon

Component: doctest coverage

Issue created by migration from https://trac.sagemath.org/ticket/32994

[mkoeppe]

comment:1

dup of #33355

[mkoeppe]

Dependencies: #33355