Add minimum versions of all requirements
This MR introduces minimum versions for all requirements in setup.cfg
in an attempt to prevent users getting into weird situations.
Specific notes:
- igwn-auth-utils 0.2.x is the first series with scitokens support
- requests-mock 1.5.0 is the first release with the pytest fixture
Other versions are nominally pinned on what works on EPEL7 (the oldest platform)