Specify version range for pytest-cov in CI

[ADBond]

pytest-cov 6.0.0 dropped support for python 3.8 support.

If we don't specify, poetry add will add it as ^6.0.0 (even when running on python 3.8). This causes dependency resolution to fail. Note that this happens even for the tests on python >= 3.9, as it finds that our general support for >=3.8 is incompatible with pytest-cov = ^6.0.0.

Output:

The current project's supported Python range (>=3.8.0,<4.0.0) is not compatible with some of the required packages Python requirement:
  - pytest-cov requires Python >=3.9, so it will not be satisfied for Python >=3.8.0,<3.9

Because no versions of pytest-cov match >6.0.0,<7.0.0
 and pytest-cov (6.0.0) requires Python >=3.9, pytest-cov is forbidden.
So, because splink depends on pytest-cov (^6.0.0), version solving failed.

For now we can add pytest-cov as >=5.0.0.

First hint of some friction with 3.8 - see #2476.