float_as_datetime doesn't handle millisecond fractionals properly

[ollz272]

See pydantic/pydantic#12027 for related issue
and #1749 for a test showing failure.

I feel like the solution is:

1. once implementing the validator (which is how i found this), have some logic in here to determine if its a millisecond/second timestamp and handle differently. Though that feels kinda nasty to do here when speedate should handle?
2. have speedate have a function that can take floats and handle properly.