Bug 1617642 Remove pip version restriction & force Python 3.8 for funsize r=jlorenzo

backports.lzma couldn't cleanly be installed because we were now using Python3.8 but hadn't specified that in the Pipfile. Also took the opportunity to remove the pip version restriction which is no longer needed.

Differential Revision: https://phabricator.services.mozilla.com/D63892

--HG--
extra : moz-landing-system : lando

taskcluster/docker/funsize-update-generator/Pipfile

@@ -1,5 +1,5 @@
 [requires]
-python_version = '3.6'
+python_version = '3.8'
 
 [packages]
 aiohttp = "*"

taskcluster/docker/pipfile-updates/scripts/update_pipfiles.sh

@@ -116,9 +116,6 @@ fi
 clone_repo
 
 ${PIP} install pipenv
-# Bug 1497162
-# Can be removed when https://github.com/pypa/pipenv/issues/2924 is released
-${PIP} install --user pip==18.0
 
 update_pipfile "${PIPFILE_DIRECTORY}"
 echo "INFO: diffing old/new Pipfile.lock into ${DIFF_ARTIFACT}"