Include a default requests.Session as part of settings that is used for HTTP requests to OSM APIs, and can be overridden using the utils.config. This should improve performance by re-using the underlying TCP connection when sending multiple requests to the same host, and allows a user of osmnx to override the session to provide client/server certificates when making request to any OSM API that uses 2-way SSL verification.

Correct formatting for imports.

Update downloader.py to fix out-of-scope variable.

Re-format code using black (for line length)

…/osmnx into use-requests-session

Switched to Ubuntu dev machine to properly run lint_test.sh, after introducing formatting issue through Github merge conflict interface.

Updated docstring to better describe how to use session. Since Session creates a persistent TCP connection that is used when requests are sent to the same host/port, remove redundant fix from #699. Unable to duplicate network error from tests after
running multiple times on Python 3.6, 3.7, 3.8, and 3.9.

Diagnosing the intermittent test failures, determined that the root cause was the server closing the connection due to inactivity. Refactored the settings to expose a dictionary of attributes that can be configured for a requests.Session that is instantiated when a call is made to _osm_network_download, _osm_geometries_download, or nominatum_request. Less elegant, but should allow users to provide authentication and 2-way SSL configuration, while also improving performance during repeated calls to the overpass API.

Properly check if key exists in session_config, so a user can only provide keys as necessary for their environment.

moving files used for local testing of different python versions