typing module requirement clobbers standard library version

As part of the setup script for the library, the typing module is specified as a requirement.

This didn't used to be part of the standard library but has been since 3.5.
I believe that we need the backport of certain features for the SDK to work correctly with its type hints.
All of these are part of the standard library version as of Python 3.7, so we should not install the external dependency if the python version is 3.7 or greater.

This is already done in 3.x (https://github.com/couchbase/couchbase-python-client/blob/master/requirements.txt#L5) but still affects the existing the 2.5.x branch.

This means that anyone who wants to install the Python SDK has to accept clobbering their standard library typing module and many build systems will actively prevent this, blocking upgrade and adoption for some users.