Drop support for EOL Python 3.4 and 3.5

[hugovk]

Also:

• Upgrade syntax for Python 3.6+: hello f-strings!
• Remove Python 2 and 3.4 imports
• Update pre-commit and apply changes
• Add pre-commit lint to GitHub Actions to make sure it stays in sync

[superdiana]

Awesome work @hugovk!! i changed the base branch to v3.0 release. There are some conflicts to resolve, mostly with supporting sms, verify and voice only with their independent classes and removing these methods from the client. Also with tests. Would you be interested on continuing the conflict resolution?

[hugovk]

Oops, wrong branch! Will update!

[hugovk]

Updated!