Issue with Python3.8 on Travis with pip 20.3.1 (stopping merges)

[ajkavanagh]

The error from #555 travis run is: https://travis-ci.org/github/juju/charm-helpers/builds/753035083

$ export ENV=pep8,py3

0.01s$ source ~/virtualenv/python3.8/bin/activate

$ python --version

Python 3.8.6

$ pip --version

pip 20.3.1 from /home/travis/virtualenv/python3.8.6/lib/python3.8/site-packages/pip (python 3.8)
before_install.1

12.80s$ sudo apt -qq update
before_install.2

$ sudo apt install -y libapt-pkg-dev
install

0.98s$ pip install tox

19.87s$ tox -c tox.ini -e $ENV

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-c41aq6p5

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'

I'll try a dummy PR to re-produce the issue from master and then the tox limit trick (mentioned in the comment in #555) to fix it.

[ajkavanagh]

#558 is a temporary fix so that work can continue; the proper solution is to get it to work with 20.3.1