Pin pip to < 20.3 as requirements.txt is broken for 20.3.1 - temporary fix

[ajkavanagh]

1. Initially just reproduce the bug in Issue with Python3.8 on Travis with pip 20.3.1 (stopping merges) #557 -- Done
2. Ultimately, use the tox fix (hopefully) to alleviate the problem: -- Done

# NOTES:
# * We avoid the new dependency resolver by pinning pip < 20.3, see
#   https://github.com/pypa/pip/issues/9187
# * Pinning dependencies requires tox >= 3.2.0, see
#   https://tox.readthedocs.io/en/latest/config.html#conf-requires
# * It is also necessary to pin virtualenv as a newer virtualenv would still
#   lead to fetching the latest pip in the func* tox targets, see
#   https://stackoverflow.com/a/38133283
requires = pip < 20.3
           virtualenv < 20.0
# NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
minversion = 3.2.0

3. Finally, work with the new resolver to sort out the dependencies so that it works. TODO in another PR

[ajkavanagh]

Reproduced failing check: https://travis-ci.org/github/juju/charm-helpers/jobs/753162791

pep8 create: /home/travis/build/juju/charm-helpers/.tox/pep8

pep8 installdeps: -r/home/travis/build/juju/charm-helpers/test-requirements.txt

ERROR: invocation failed (exit code 1), logfile: /home/travis/build/juju/charm-helpers/.tox/pep8/log/pep8-1.log

================================== log start ===================================

Ignoring ipaddress: markers 'python_version < "3.0"' don't match your environment

Ignoring PyYAML: markers 'python_version >= "3.0" and python_version <= "3.4"' don't match your environment

Ignoring Jinja2: markers 'python_version >= "3.0" and python_version <= "3.4"' don't match your environment

Ignoring Jinja2: markers 'python_version == "2.7" or python_version == "3.5"' don't match your environment

Collecting git+https://git.launchpad.net/ubuntu/+source/python-distutils-extra (from -r /home/travis/build/juju/charm-helpers/test-requirements.txt (line 3))

  Cloning https://git.launchpad.net/ubuntu/+source/python-distutils-extra to /tmp/pip-req-build-jugttyff

Requirement already satisfied: pip in ./.tox/pep8/lib/python3.8/site-packages (from -r /home/travis/build/juju/charm-helpers/test-requirements.txt (line 4)) (20.3.1)

Collecting amulet

  Downloading amulet-1.21.0.tar.gz (30 kB)

Collecting coverage>=3.6

  Downloading coverage-5.3.1-cp38-cp38-manylinux2010_x86_64.whl (245 kB)

Collecting distro-info

  Downloading distro-info-0.10.tar.gz (5.7 kB)

ERROR: Requested distro-info from https://files.pythonhosted.org/packages/91/e0/ccd731d32238bc18bb8ac80353b4028394f80d5ff7db0f93a5ba488690d3/distro-info-0.10.tar.gz#sha256=3c5c2c800e84f36e56e4c59b09981eae357127494f9eb391712888c3f7ac2916 (from -r /home/travis/build/juju/charm-helpers/test-requirements.txt (line 13)) has different version in metadata: '0.0.0'

[gnuoy]

LGTM