Ideas for breaking changes in v1 release

[carver]

eth-keys is relatively stable. The next breaking release should probably jump to v1. Let's compile ideas for breaking changes we might want to make:

• consider high-s signatures invalid by default (maybe with an option to explicitly permit them) -- see #62
• ...?

### Tasks
- [ ] Audit!

[pipermerriam]

Need to fix how backends work so that they are classes rather than modules so that they can be pickled. This may not require a breaking change.

[carver]

Drop py3.5 support (maybe even 3.6?)