doesn't preserve the zero sign (if TANH_PRESERVES_ZERO_SIGN define is 0).

Gay's dtoa.c (but not for current versions of Python's dtoa.c).
Thanks Rick Regan for finding and reporting this.

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.