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.