Remove greenlet dependency

[mrcljx]

The greenlet>=2.0.1 requirement is forced on all projects importing this package, but it's actually not imported (except from tests for which it already is listed in testsrequire.py).

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

[xzkostyan]

@randomowo do you have any idea why we need this specific version?

[coveralls]

coverage: 85.736%. remained the same when pulling 2aba46d on mrcljx:patch-1 into 9545a17 on xzkostyan:master.