This is probably an implicit standard via the implementation details of distutils/setuptools/pip that could stand to be explicitly defined.
And like many things in this area, it’s complicated 
The de facto standard (implemented by setuptools) is to convert all running characters outside [A-Za-z0-9.-] to a single underscore, and covert to lowercase. User can specify effectively anything in setup.py.
But Core Metadata (linked above) says an extra name (quote) Must be a valid Python identifier—in other words, can’t contain - and ., and can’t start with a digit, etc.[1] This is in direct conflict to setuptools’s implementation, which would happily produce invalid extra names according to Core Metadata. (I’m not sure if wheel implements additional logic to block those names from being written into a wheel.)
So if we’re going to have a standard on this, this would be a good opportunity to fix the available character set. I’m not sure what the best rule is here considering backwards compatibility. The general form is easy to agree on, but there are various edge cases like whether the name can begin with a digit or a non-alphanumeric character.
The normalisation rule is probably good enough as-is, although I’d personally hope we can change to a rule similar to PEP 503 to avoid user confusion.
[1]: Since the rule was written pre-Python 3, I assume non-ASCII characters are also out.