Release 3.3.0

This release includes many usability and stability fixes:

• Added get_by function to inventory API to return a single object by query.
• Added CumulocityApp class to module c8y_tk.app which allows working with Cumulocity interactively, e.g. in a
  Jupyther notebook. It will deal with environment variables just like the other connection helpers but will also
  ask interactively for missing info, e.g. a second factor with 2FA. It also integrates well with the
  c8y-go-cli tool.
• Added c8y_tk.app packages with SubscriptionListener class to ease development of multi_tenant microservices
  which need to act on all subscribed tenants or on added/removed subscriptions alike.
• Adding debug logs to base API as default urllib3 logs are not helpful for our purpose.
• Single and multi tenant applications now automatically set the application_key property from the standard
  APPLICATION_KEY environment variable.
• Added get_count function to Operations API.
• Added as_values parameter to get_all and select functions of the Inventory, DeviceInventory,
  DeviceGroupInventory, Events, Alarms, Users, Operations, and AuditRecords API.
• Added code coverage reporting to test target for invoke.
• Updated as_tuple for complex objects as well as the as_values parameter for select
  and get_all functions to work with strings or 2-tuples. The use of a dictionary
  was removed as dictionaries don't define an order.
• Added as_values parameter to the Measurements API select and get_all functions.
• Adding c8y_tk.analytics package with to_numpy, to_series and to_data_frame functions to
  ease incorporating Cumulocity data into standard analytics pipelines.