Improved config mode(s) handling, raise for status (responses), better.... things?

• Add underscores to EOS config prompt matching
• Actually fixed on_close methods that I could have sworn were fixed.... gremlins! (was sending prompt pattern instead of a return char... for copypasta reasons probably)
• No longer "exit" config mode... given that send_command like methods already check to ensure they are in the right priv level there is no reason to exit config mode... just leave it when you need to. Should be a minor speed up if using send_configs more than once in a row, and otherwise should be basically exactly the same.
• For NetworkDrivers we no longer set the channel prompt pattern depending on the priv level -- it is now always the combined pattern that matches all priv levels... this should make doing manual things where you change privileges and don't use scrapli's built in methods a little easier. Scrapli still checks that the current prompt matches where it thinks it should be (i.e. config mode vs privileged exec) though, so nothing should change from a user perspective.
• Improve (fix?) the abort config setup for IOSXR/Junos
• Add more helpful exception if ssh key permissions are too open
• Convert PrivilegeLevel from a namedtuple to a class with slots... better for typing and is also mutable so users can more easily update the pattern for a given privilege level if so desired
• Minor clean up stuff for all the core platforms and network driver, all internal, mostly just about organization!
• Add "configuration_exclusive" privilege level for IOSXRDriver, add "configuration_private" and "configuration_exclusive" for JunosDriver, modify some of the privilege handling to support these modes -- these can be accessed by simply passing privilege_level="configuration_exclusive" when using send_configs method
• Add support for configuration sessions for EOS/NXOS. At this time sessions need to be "registered" as a privilege level, and then are requestable like any other privilege level, and can be used when sending configs by passing the name of your session as the privilege level argument for send config methods
  Add a space to EOS prompts -- it seems its very easy to add one to the prompts and scrapli did not enjoy that previously!
• Give users the option to pass in their own privilege levels for network drivers, and also throw a warning if users try to pass comms_prompt_pattern when using network drivers (as this should all be handled by priv levels)
• Created MultiResponse object to use instead of a generic list for grouping multiple Response objects
  Added raise_for_status methods to Response and MultiResponse -- copying the requests style method here to raise an exception if any elements were failed
• BUGFIX: fixed an issue with IOSXEDriver not matching the config mode pattern for ssh pub key entries.