use `max_age` instead of `cache_timeout` (for flask > 2.0)

[BuckarooBanzay]

fixes #56

disclaimer: i'm a python-newbie, feedback appreciated

[rubenwardy]

The serverlist has "flask>=1.1.0", so flask 2 may not be totally supported

But the changes look good, the documentation confirms that this is the correct way to do this in flask 2.

[sfan5]

Please adjust requirements.txt too then.

The serverlist has "flask>=1.1.0", so flask 2 may not be totally supported

I think the flask~=1.1.0 would have been the correct syntax to stay at a compatible major version.