Use 'https' instead of 'http' for dnspython.org (#883)

Use 'https' instead of 'http' for dnspython.org (#883)

These days, 'https' should be used instead of 'http' for almost
anything, and Coverity is warning that dnspython.org URL in 'setup.cfg'
uses 'http'.

This patch changes the use of 'http' to 'https' on setup.cfg and
documentation, where it does not affect module code, tests or examples.