Remove gevent as direct dependency

When installing `dramatiq[watch]`, this installs `watchdog_gevent`, which then installs `gevent` and its dependencies.

This means someone not using gevent, but developing with dramatiq ends up with more things installed than required. Not a big deal, but could be improved.

In fact the README says

> This will automatically import the best observer for the current platform, preferring a [gevent](http://www.gevent.org/)-based observer **if gevent is installed** and the threading module has been monkeypatched

In fact, using this library means gevent is **always** installed, because it is a dependency.

We should remove gevent as a dependency, so someone working with `dramatiq[watch]` doesn't get it installed, but someone working with `dramatiq[gevent,watch]` does.

And probably remove the warning here

https://github.com/Bogdanp/watchdog_gevent/blob/8f70b860832c907e5fcd4d70e4d1d5abed2e4407/watchdog_gevent/__init__.py#L9-L10

So that non-gevent users fallback to regular watchdog without any warnings.