Improve typing support for the files parameter

[florimondmanca]

When files is passed from a variable, mypy raises an error due to us using a Dict with union values:

files = {"example": ("file name", "file content")}
_ = httpx.post("...", files=files)

error: Argument "files" to "post" has incompatible type "Dict[str, Tuple[str, str]]"; expected "Optional[Dict[str, Union[Union[IO[str], IO[bytes], str, bytes], Tuple[Optional[str], Union[IO[str], IO[bytes], str, bytes]], Tuple[Optional[str], Union[IO[str], IO[bytes], str, bytes], Optional[str]]]]]"
note: "Dict" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
note: Consider using "Mapping" instead, which is covariant in the value type

This PR fixes this small UX bug. :-)

(Progressing on #650 would allow us to catch more of these earlier.)

[tomchristie]

👍