[3.13] gh-130959: Reject whitespace in fractions, in pure Python `fromisoformat()` (#130962) (#131076)

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

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`).

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>
(cherry picked from commit 33494b4d0dafc34ff4f1c118b7b3b5d8de3dd0f4)

Co-authored-by: Michał Górny <mgorny@gentoo.org>