build(pip): Enable the new 2020 resolver (#21507)

This reverts commit c9f3294e9e65787a75538994353141849b198070 and starts using `pip` `20.3.0` which has the new, strict resolver turned on by default.

docker/Dockerfile

@@ -79,6 +79,7 @@ RUN set -x \
   " \
   && apt-get update \
   && apt-get install -y --no-install-recommends $buildDeps \
+  && python -m pip install --upgrade 'pip>=20.3.0' \
   && pip install -r /tmp/dist/requirements.txt \
   && mkdir /tmp/uwsgi-dogstatsd \
   && wget -O - https://github.com/eventbrite/uwsgi-dogstatsd/archive/filters-and-tags.tar.gz | \