Upgrade to OpenApi3.1

What's Changed

OpenApi 3.1 is divergent from semver and has breaking changes. For more information on 3.1,
https://spec.openapis.org/oas/v3.1.0
https://www.openapis.org/blog/2021/02/16/migrating-from-openapi-3-0-to-3-1-0

3.1 changes that effect flask-rebar's implementation:

• removal of "nullable" to now allow the type keyword to define multiple types for a schema with an array

• ACSBP-3718 Update to use openapi 3.1 by @auramix in #284

Other quality-of-life fixes:

• install mypy + update dev deps + update pre commit + begin type hinting by @dekim24 in #292

Full Changelog: v2.4.1...v3.0.0