Fix mypy errors (#229)
Fix mypy errors (#229)

In particular, the following errors are fixed in this commit:

* Don't return False in __exit__

Returning a literal causes a mypy error when combined with the
`typing.Optional[bool]` type hint. Furthermore, exception handling is
the same when returning `False` and when returning `None` (the
exception is re-raised). Therefore, it's simpler to remove the return
statement and change the type hint to `None`.

* Correctly initialize nested tuple

Tuples of length 1 should be initialized with a trailing comma to be
properly interpreted.

* Pass correct type to use_context() in test

* Add type annotations for test helper functions

Since we have `disallow_untyped_calls = True` in our mypy config for
tests, we must add type annotations to any function that is called
from a test.


Addditionally, bump minimal mypy version to 0.740 to consistently reproduce these errors.

open-telemetry/opentelemetry-python-contrib,open-telemetry/opentelemetry-python-contrib,open-telemetry/opentelemetry-python-contrib