[2.7] closes bpo-32997: Fix REDOS in fpformat (GH-5984)

[2.7] closes bpo-32997: Fix REDOS in fpformat (GH-5984)

The regex to decode a number in fpformat is susceptible to catastrophic backtracking. This is a potential DOS vector if a server is using fpformat on untrusted number strings.

Replace it with an equivalent non-vulnerable regex. The match behavior of the new regex is slightly different. It captures the whole integer part of the number in one group, Leading zeros are stripped off later.