Typing module change makes SDK uninstallable by default on Python 3.5/3.6

Raised this as PYCBC, but the outcome might be a docs change, just needed somewhere to capture it.

A few days ago, a new version of the typing module was released that prevented installations on versions 3.5 or older - https://github.com/python/typing/commit/0b0e24801f8ec5322ee84d27024cae1805449396, as part of typing 3.7.4.2.

This is problematic for us, as the SDK conditionally installs this dependency if the version of python is lower than 3.7.
This makes the SDK uninstallable by default on Python 3.5 and 3.6, due to dependency mismatch.

The workaround is to install typing == 3.7.4.1 before installing the Couchbase SDK, as the dependency in the SDK will be satisfied by 3.7.4.1, which accepts any version but will default to latest.

The Python team have acknowledged this breaking change as a problem in https://github.com/python/typing/issues/739, so looks like the Python core team will probably revert it.
Until then though we may have users who cannot install the SDK, so we probably need to cover this prominently in the release notes or something

We may also want to consider some kind of 'future proofing' in this area for the SDK going forward.