pbr tests fail with virtualenv-20+

[Michał Górny]

When running `tox -e py37`, I get the following failure:

```
==============================
Failed 1 tests - output below:
==============================

pbr.tests.test_packaging.TestRequirementParsing.test_requirement_parsing
------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/pbr/.tox/py37/lib/python3.7/site-packages/fixtures/fixture.py", line 197, in setUp
    self._setUp()

      File "/tmp/pbr/pbr/tests/test_packaging.py", line 186, in _setUp
    virtualenv.create_environment(path, clear=True)

    AttributeError: module 'virtualenv' has no attribute 'create_environment'

Captured traceback-1:
~~~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/pbr/.tox/py37/lib/python3.7/site-packages/fixtures/fixture.py", line 197, in setUp
    self._setUp()

      File "/tmp/pbr/pbr/tests/test_packaging.py", line 186, in _setUp
    virtualenv.create_environment(path, clear=True)

    AttributeError: module 'virtualenv' has no attribute 'create_environment'

During handling of the above exception, another exception occurred:

    Traceback (most recent call last):

      File "/tmp/pbr/.tox/py37/lib/python3.7/site-packages/fixtures/fixture.py", line 208, in setUp
    raise SetupError(details)

    fixtures.fixture.SetupError: {}

Captured stdout:
~~~~~~~~~~~~~~~~
    Initialized empty Git repository in /tmp/tmpnx2bzw9_/tmpvu01g7fg/test_reqparse/.git/

[master (root-commit) 0e18ca0] test commit
 4 files changed, 19 insertions(+)
 create mode 100644 4
 create mode 100644 requirements.txt
 create mode 100644 setup.cfg
 create mode 100644 setup.py
```

The issue disappears if I force virtualenv < 20.