Use only one global warning.catch_warnings() context, instead of two local
contexts. Improve also the explaination why the function uses a local import.

With Raymond’s approval.

This was done in r80805 (#7908) and erroneously brought back by the
distutils revert.  This commit removes more code than the original,
which was uncomplete.  There is no NEWS entry, like in r80805.

unpickling them produced nonsensical results.

its low-level signal handler.  Patch by Hallvard B Furuseth.

(ISLOWER()/ISUPPER()/etc) have now been removed: use Py_ISLOWER() etc
instead.

Merge the doc for assertTrue/False, assert[Not]AlmostEqual, assert[Not]RegexpMatches, rephrase a couple of paragraphs, and remove redundant doc about the msg arg.