gh-130959: Reject whitespace in fractions, in pure Python `fromisoformat()` (GH-13096...

gh-130959: Reject whitespace in fractions, in pure Python `fromisoformat()` (GH-130962) (GH-131076) (#131086)

Fix the pure Python implementation of `fromisoformat()` to reject any
non-digit characters, including whitespace, in the fractional part
of time specification.  This makes the behavior consistent with the C
implementation, and prevents incorrect parsing of these fractions
(e.g. `.400 ` would be misinterpreted as `.04`).

(cherry picked from commit 33494b4d0dafc34ff4f1c118b7b3b5d8de3dd0f4)
(cherry picked from commit 27fd328cf68e5d812cdb7f9533a16dd68d911c54)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Michał Górny <mgorny@gentoo.org>