Specify minimum dependencies

[daenney]

@singingwolfboy Should we consider specifying some minimum dependencies in requirements.txt at this point? Right now there's a fairly wide range of allowed versions, which aren't likely to all work well together.

It can also cause some confusion like in #236, where at least the dependency wouldn't have been able to be satisfied returning an error to the user instead of failing at runtime.

For example, I would suggest we use:

• Flask >= 1.0.0
• requests-oauthlib >= 1.2.0

We should probably do this as part of #231.