Pin greenlet dependency to address install oddities

[MJJoyce]

Poetry keeps attempting to downgrade greenlet to an older version when installing GUI.

❯ poetry install

...

  • Updating greenlet (1.1.3 -> 0.4.16): Failed

Notably, it does this when installing Core and GUI into the same virtual environment. Released version of core specified the older 0.4.16 pin. However, the up-to-date versions pin greenlet to 1.1.3. Updating GUI's lock file after installing Core doesn't avoid this inconsistency either.

We can avoid this by adding greenlet as a dependency in GUI and updating the lock file.