`dig pipenv.org ns` currently shows no nameserver for that domain;
pipenv.pypa.io is the new home for documentation, so use that
everywhere (contributors guides, `pipenv --envs` output).
The canonical Pipenv documentation is now at pipenv.pypa.io.
Also, the canonical GitHub repositories for Pipenv and
Requests have changed, and some other communications links
(Twitter, mailing list, "thank you") were no longer
operational. This commit updates those and clarifies that
Pipenv is a project maintained by the PyPA.
Fixes#4137.
Signed-off-by: Sumana Harihareswara <sh@changeset.nyc>