Creating a python3 virtualenv fails on bionic

[sabaini]

Creating a python3 virtualenv with pip_create_virtualenv() fails, as the virtualenv binary isn't part of python3-virtualenv pkg on bionic (it seems it's present in this package in focal though)

2020-11-11 16:06:08 [ERROR]         self._reemit(event_path)
2020-11-11 16:06:08 [ERROR]       File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 723, in _reemit
2020-11-11 16:06:08 [ERROR]         custom_handler(event)
2020-11-11 16:06:08 [ERROR]       File "./src/charm.py", line 41, in on_install
2020-11-11 16:06:08 [ERROR]         self.helper.create_venv()
2020-11-11 16:06:08 [ERROR]       File "/var/lib/juju/agents/unit-juju-lint-0/charm/lib/lib_jujulint.py", line 130, in create_venv
2020-11-11 16:06:08 [ERROR]         pypkg.pip_create_virtualenv(Paths.VENV_DIR)
2020-11-11 16:06:08 [ERROR]       File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/charmhelpers/fetch/python/packages.py", line 154, in pip_create_virtualenv
2020-11-11 16:06:08 [ERROR]         subprocess.check_call(['virtualenv', venv_path])
2020-11-11 16:06:08 [ERROR]       File "/usr/lib/python3.6/subprocess.py", line 306, in check_call
2020-11-11 16:06:08 [ERROR]         retcode = call(*popenargs, **kwargs)
2020-11-11 16:06:08 [ERROR]       File "/usr/lib/python3.6/subprocess.py", line 287, in call
2020-11-11 16:06:08 [ERROR]         with Popen(*popenargs, **kwargs) as p:
2020-11-11 16:06:08 [ERROR]       File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
2020-11-11 16:06:08 [ERROR]         restore_signals, start_new_session)
2020-11-11 16:06:08 [ERROR]       File "/usr/lib/python3.6/subprocess.py", line 1364, in _execute_child
2020-11-11 16:06:08 [ERROR]         raise child_exception_type(errno_num, err_msg, err_filename)
2020-11-11 16:06:08 [ERROR]     FileNotFoundError: [Errno 2] No such file or directory: 'virtualenv': 'virtualenv'
2020-11-11 16:06:08 [ERROR]     2020-11-11 15:06:06 ERROR juju.worker.uniter.operation runhook.go:136 hook "install" (via hook dispatching script: dispatch) failed: exit status 1
2020-11-11 16:06:08 [ERROR]     2020-11-11 15:06:06 INFO juju.worker.uniter resolver.go:143 awaiting error resolution for "install" hook
2020-11-11 16:06:08 [ERROR]     

ubuntu@juju-4e2360-0:~$ dpkg -L python3-virtualenv
/.
/usr
/usr/lib
/usr/lib/python3
/usr/lib/python3/dist-packages
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info/PKG-INFO
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info/dependency_links.txt
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info/entry_points.txt
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info/not-zip-safe
/usr/lib/python3/dist-packages/virtualenv-15.1.0.egg-info/top_level.txt
/usr/lib/python3/dist-packages/virtualenv.py
/usr/share
/usr/share/doc
/usr/share/doc/python3-virtualenv
/usr/share/doc/python3-virtualenv/changelog.Debian.gz
/usr/share/doc/python3-virtualenv/copyright
ubuntu@juju-4e2360-0:~$ apt-cache policy python3-virtualenv
python3-virtualenv:
  Installed: 15.1.0+ds-1.1
  Candidate: 15.1.0+ds-1.1
  Version table:
 *** 15.1.0+ds-1.1 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status