Ignore mypy errors

[chlowell]

Ignoring a few mypy errors in authentication and authentication_async:

• self._update_headers(request.http_request.headers, token) raises an error because the type variable HTTPRequestType doesn't have a headers attribute. Tracked by HTTPRequestType has no attribute "headers" #5796.
• await self._credential.get_token(*self._scopes) raises an error because the credential's Protocol defines get_token as returning a string. At runtime however, self._credential will be an async credential and get_token will return a coroutine.
• await self.next.send(request) raises an error because self.next is typed Optional[Any]. Looks like this should be something else, e.g. Optional[AsyncHTTPPolicy]. Tracked by AsyncHTTPPolicy's next attribute is Optional[Any] #5797.

[adxsdk6]

Can one of the admins verify this patch?

[chlowell]

/azp run azure-sdk-for-python - client

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