Using ContextVars to allow dominate to work within async contexts. Added
unit tests to ensure code works as expected.

 - Added .venv and .envrc to .gitignore (I use direnv and venv to keep
   my python environments isolated - I hope this is okay!)
 - Removed print statements I left in dom_tag during debugging
 - Replaced global incrementing int with UUID for contextvar ID
   generation - this zeroes the risk of race-hazards/collisions
 - _get_thread_context now returns a tuple vs. a hash of a tuple.
   Functionally not much changes - the underlying dictionary will still
   use the same hashing function but the only difference is that _if_
   there is a collision, the dictionary will still be able to return the
   correct element