Allow for "pdb" module to be rewritten

[blueyed]

Noticed this with tests for pdb++.

[blueyed]

Note that it still tries to rewrite it (

pytest/src/_pytest/assertion/rewrite.py

Lines 233 to 234 in c1361b4

	self._warn_already_imported(name)
	self._must_rewrite.update(names)

), and might succeed (likely due to the "pdb" module being re-imported explicitly in the tests).

[nicoddemus]

Missing changelog, otherwise LGTM. 👍

[nicoddemus]

Needs rebase once #5923 is merged

[blueyed]

Missing changelog, otherwise LGTM. 

Isn't that just noise? (quite internal isn't it?).

[nicoddemus]

Isn't that just noise? (quite internal isn't it?)

Fair enough. 👍