3.X: Improve install when access to github is blocked

Problem
To install the Python SDK, libcouchbase is downloaded from github and built from source.
Some companies block githhub, this means the install has to be done using a local installed libcouchbase. Unfortunately these steps are incomplete.

To do a install using the local libcouchbase

PYCBC_BUILD=DISTUTILS PATH=/usr/local/bin:$PATH python3 -m pip install couchbase

This step is missing:

cp /usr/lib64/libcouchbase.so.8.0.8 /usr/local/lib/python3.8/site-packages/couchbase_core/libcouchbase.so.8

Expectation

1. For the install steps to be correct
2. There should not be the need to manually copy files around after a install.

Ideally we would go with 2.