Set version number to 2.0.1, get back in sync with tags

We have a 2.0.0 tag in github which points to code where the version claims
to be 1.0.0:

https://github.com/alphagov/backdrop-collector/blob/2.0.0/backdrop/collector/__init__.py

We definitely have code which specifies 2.0.0 as its dependency.

Upversion to 2.0.1 so we can make a new, correct, tag.