fix: Change asynchronous dependencies for tox
fix: Change asynchronous dependencies for tox

Noticed WARNINGS in Travis CI output:

WARNING:test command found but not installed in testenv

Looks like the test environments expected to use `requirements-test.txt`
with one extra package, `gevent`. Tests should be able to run without
issues.

PabloLefort/graphitesend,daniellawrence/graphitesend