www/py-tvdb_api: Add missing dependencies

tvdb_api 2.0 added dependencies on requests and request-cache [1][2]. Without
them, multimedia/py-tvnamer fails at runtime:

"DistributionNotFound: The 'requests' distribution was not found and is required by tvdb-api"
"DistributionNotFound: The 'requests_cache' distribution was not found and is required by tvdb-api"

Add requests and request-cache to RUN_DEPENDS accordingly.

While I'm here, level up port compliance:

• Update Python version restriction (2.0 supports Python 3)
• Add TEST_DEPENDS and test target
• Add LICENSE / LICENSE_FILE

[1] https://github.com/dbr/tvdb_api/commit/ea7f2d57
[2] https://github.com/dbr/tvdb_api/commit/f05a327d

portlint: OK (looks fine.)
testport: OK (poudriere, 12amd64, Python 2.7/3.6)
unittest: OK (42 passed, 3 skipped in 45.39 seconds)

Reported by: Georg Huber (via email)
Approved by: portmgr (blanket)