Revert "[FIX] remove conftest for externals"

[Remi-Gau]

Reverts #4306
Closes #4305

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[bthirion]

I'm a bit lost... the Numpy2 issue is something else ?

[Remi-Gau]

I'm a bit lost... the Numpy2 issue is something else ?

• Yes the numpy 2 issue (numpy 2 test workflow failures #4309) of the tests currently failing is something else (and actually probably a pandas 3.0 thing: still need to investigate).

• I had opened Tests failing on main with pytest 8.1.0 #4305 and "fixed" because all of tests were failing when using the latests release of pytest (8.1.0). But @emdupre followed up and it seems that the latest version of pytest was bugged and got yanked from pypi and that we should not have needed the "fix" in the first place.

This PR only reverts this.

[emdupre]

This LGTM, thx @Remi-Gau !