Cryptojwt

- add `ignore_errors_period` option to `KeyBundle` for setting optional error hold down period
- fix issue with key activation/inactivation for `KeyBundle` remote sources

- Fix bug in iqmp calculation (fix from c00kiemon5ter)
- More code reformatting
- Check formatting via tox
- Use Python 3.8 for quality testing

- Try to have the same support for different cryptographic algorithm.
- Added black and made it work with isort.
- Support for persistent storage in KeyJar.

The background to the changes in this release is the need to be able to store dynamic information in a persistent way.
While doing that work splitting the KeyJar class into KeyJar and KeyIssuer became worthwhile.
There was also an inconsistency when it came to the parameter name for the issuer identifier.
Sometimes it was issuer sometimes owner. Changed that into always be issuer_id.

- Re-read KeyBundle local files if modified
- Honor `If-Modified-Since` header on KeyBundle HTTP fetch
- Add key wrapping/unwrapping utility functions