rpnChilada 8.0.0 - Unit Conversions Revamped

The unit conversion code has been heavily refactored and works much better now.

Added the 'base_units' and 'dimensions' operators, mostly for testing purposes.

Added '_dump_conversions' and '_dump_cache', also for testing purposes.

rpnChilada is now smart enough to recognize when an OEIS request has failed,
and to ignore the cached result stored as a result. If it detects that the
cached value is empty, it will perform the request again and recache the
result.

Help now supports units and constant operators after way too long. Filling in
the help info for the units and constant operators, along with all the existing
help info that's missing, will take a while, and is continuing.

rpnChilada has officially dropped Python 2 support. I rarely tested it anyway.

Added 'wind_chill' and 'heat_index' operators.

The unit tests now confirm that aliases do not collide with other reserved
words. The alias creation for generated types has also been cleaned up.

The astronomy functionality has been refactored to support migrating to the
skyfield library from pyephem.