Fix checksumming in Python3; add more fetch tests (#3941)

* Checksum code wasn't opening binary files as binary.

- Fixes Python 3 issue where files are opened as unicode text by default,
  and decoding fails for binary blobs.

* Simplify fetch test parametrization.

* - add tests for URL fetching and checksumming.
- fix coverage on interface functions in FetchStrategy superclass
- add some extra crypto tests.