Permit variable-level scopes for client fixture.

[faelin]

Sometimes a user might not want the client fixture to be function-scoped. For example, another fixture may rely on the connection info of the client, such as port-number, and might want that data to remain static throughout a session, or might need that data in a session-scoped fixture.

Chore that needs to be done:

• Add newsfragment pipenv run towncrier create [issue_number].[type].rst

Types are defined in the pyproject.toml, issue_number either from issue tracker or the Pull request number