tests: use pytest instead of nose and other small test-related stuff
related to swh-environment#1261 (closed)
also:
- tests: assertEquals -> assertEqual to clear deprecation warnings.
- tests: rename TestedMerkleLeaf and TestedMerkleNode classes so pytest does not think they are test classes.
- add a simple tox file
- add .tox to gitignore
Test Plan
run 'tox'
Migrated from D549 (view on Phabricator)