normalize numeric literals (#454)

Fixes #452

I ended up making a couple of other normalizations to numeric literals
too (lowercase everything, don't allow leading or trailing . in floats,
remove redundant + sign in exponent). I don't care too much about those,
so I'm happy to change the behavior there.

For reference, here is Python's grammar for numeric literals:
https://docs.python.org/3/reference/lexical_analysis.html#numeric-literals