This updates CredentialChckerTool to subclass the newer BaseTool.
The original implementation had some regex patterns that didn't survive

the later stages of development, when combining into a single verbose

regex. Any pattern that expected a space (such as PGP private key

blocks) failed to match. This is now fixed.
The GitHub token checks have been greatly improved. As of April 2021,

GitHub introduced a new token format that can more easily be scanned,

and contains a checksum as the final 6 characters of the token, allowing

a scanner to detect real vs. fake tokens (though not active vs.

inactive). We now support enhanced validation for these GitHub tokens,

and have logic in place to allow for enhanced validation for future

tokens as well.
Most previous token types have been updated for modern token formats and

to reduce false-positives by preventing the tokens from being embedded

in other strings.
Token support has been added for:

Asana
Discord
Dropbox
Google Cloud Platform
JSON Web Tokens
Mailchimp
Mailgun
NPM
PyPI
Slack (WebHooks)
SSL Certificates
Stripe
Twilio

Unit tests pass on Python 2.7 and 3.x.
Extensively tested the new GitHub token validation logic. I've tested this

with a number of real-world tokens, along with garbage tokens, ensuring that

the real ones always validated and matched.