Codecov CI action should install package in dev mode

[hombit]

In current main, codecov action installs the user's package with pip install .. It looks like sometimes it may cause some issues with pytest, see this failed action (corresponding workflow):
https://github.com/lincc-frameworks/pandas-ts/actions/runs/7785027534/job/21226701366?pr=20

I'm not sure why it happens, it may be issue with my project. Shallow googling showed that maybe it is because pytest is unhappy with pip install which may leave some artifacts in the working directory. I solved the issue on my side changing both coe installation lines to

pip install -e .
pip install -e .[dev]

[hombit]

I have the same problem with the smoke test

https://github.com/lincc-frameworks/pandas-ts/actions/runs/7795789421/job/21259304324#step:6:1

[delucchi-cmu]

Based on my (very limited) experimentation, this looks more related to removing the tests argument to pytest, which YOU ASKED FOR.

#249

https://github.com/lincc-frameworks/pandas-ts/pull/24/files

[hombit]

I think i have all these troubles because I added doctests, so no pytest is asked to look into both src/ and tests/ so it imports the code twice