Outdated RFC link in README.rst and elsewhere

[jennifer-richards]

Describe the bug
The README.rst and other files contain an outdated link to RFC 6749.

To Reproduce
Visit the github page and note that the URL for the statement about being rfc-compliant leads to a URL at tools.ietf.org, a now deprecated domain. Search the source code and find other mentions of tools.ietf.org.

Expected behavior
The link should point to the current location of the resource, https://www.rfc-editor.org/info/rfc6749 (for the info page with various formats and metadata about the RFC) or https://www.rfc-editor.org/rfc/rfc6749.html (for a direct link to the HTML version).

Version
current main branch

• I have tested with the latest published release and it's still a problem.
• I have tested with the master branch and it's still a problem.

Additional context
The tools.ietf.org site has been retired / deprecated and, while it redirects to a working location, should no longer be used.

[n2ygk]

A PR to fix this would be appreciated!