Fix handling of None values in timeout tuple type hint

[florimondmanca]

Fixes an issue with our TimeoutTypes that makes mypy complain if users pass None's as part of a timeout tuple (although None are accepted, meaning "no timeout"):

# example.py
import httpx

_ = httpx.Timeout((1.0, None, None, None))

$ mypy example.py
example.py:3: error: Argument 1 to "Timeout" has incompatible type "Tuple[float, None, None, None]"; expected "Union[float, Tuple[float, float, float, float], Timeout, None]"
Found 1 error in 1 file (checked 1 source file)

[yeraydiazdiaz]

LGTM!