Drop table support in python

[szehon-ho]

After create-table is supported in: #1216, it makes sense to implement drop table.

Unless I missed it somwhere, it doesn't seem it's supported now as per: https://github.com/apache/iceberg/blob/master/python/iceberg/hive/hive_tables.py#L37

[szehon-ho]

I would be happy to give this a stab, but wanted to check if anyone is working on it? @rdblue @rymurr thanks

[rymurr]

Hey @szehon-ho no one that I am aware of is working on it and your contributions would be very welcome! Shout if you have any questions.

[szehon-ho]

Hi @rymurr i pushed my PR, if you have time to take a look.

I tested manually on real Iceberg table. Verified that table metadata is dropped, and that all the expected data/metadata files are gone if purge=true.

The 3.7 tox test failures were broken before my change actually.