Narrowing inside generic functions with constraints

[JelleZijlstra]

This currently produces a bunch of false-positive errors:

$ python -m pyanalyze -c '''from typing import TypeVar
T = TypeVar("T", None, str)
def f(source: T) -> T:
    if source is None:
        return source
    if source.startswith("x"):
        return source[1:]
    return source
'''

None has no attribute 'startswith' (code: undefined_attribute)
In <code> at line 6
   3: def f(source: T) -> T:
   4:     if source is None:
   5:         return source
   6:     if source.startswith("x"):
             ^
   7:         return source[1:]
   8:     return source

<uninitialized> is not callable (code: not_callable)
In <code> at line 7
   4:     if source is None:
   5:         return source
   6:     if source.startswith("x"):
   7:         return source[1:]
                     ^
   8:     return source