2021.07.30a1 Pre-Release; commandeer, a sneaky Easter egg, unblock some work on scrapli netconf!

• Added "% Unavailable command" to EOS failed_when_contains
• Moved core platform failed_when_contains to base to not have to duplicate them in sync and async platforms
• Add file_mode to the enable_basic_logging function, can now choose "append" or "write" for logfile
• Add channel_log_mode to the base driver arguments; you can now choose "append" or "write" for this as well!
• Improve reading until prompt methods; no longer use re.search on the entire received byte string, now only checks
  for prompt on the last N chars where N is governed by the base channel args comms_prompt_search_depth attribute..
  . this fixes an issue where scrapli could be wayyyyyy slow for very very large outputs (like full tables show bgp)
• Fix bug (or just terrible initial idea!?) in asynctelnet that reset a timer back to a very small value that was used
  for testing; most people shouldn't have noticed an issue here, but if you had slow devices this could cause
  issues that "looked" like an authentication issue due to scrapli not having responded to all telnet control
  characters before punting to auth
• Added commandeer to driver object; this is used to "commandeer" an existing connection but treat it like the new
  connection object (prompt patterns, methods, etc.) -- generally this would be used for using GenericDriver to
  connect to a console server, then "commandeering" that connection and turning it into an IOSXR/IOSXE/etc.
  connection object so you have all the "normal" behavior of scrapli
• Add missing timeout on the asynctelnet open method
• Add py.typed to hopefully do typing more correctly :P