bpo-46852: Rename float.__set_format__() to float.__setformat__() (GH-31558)

Rename the private undocumented float.__set_format__() method to
float.__setformat__() to fix a typo introduced in Python 3.7. The
method is only used by test_float.

The change enables again test_float tests on the float format which
were previously skipped because of the typo.

The typo was introduced in Python 3.7 by bpo-20185
in commit b5c51d3dd95bbfde533655fb86ac0f96f771ba7b.