Add support for pytest5

[AceFire6]

Closes #142

[AceFire6]

It seems the warning being shown changes the output the test expects.

In the file test.test_testmon.TestmonDeselect#test_tlf::261 the actual output is:

'===================== 1 failed, 1 warnings in 0.03 seconds ====================='

and what is expected is:

'===================== 1 failed in 0.03 seconds ====================='

[blueyed]

1. needs to test pytest5 explicitly on CI

2. the warning

   =============================== warnings summary ===============================
   /home/travis/build/tarpas/pytest-testmon/.tox/py37-pytestmaster/lib/python3.7/site-packages/_pytest/config/__init__.py:758
   /home/travis/build/tarpas/pytest-testmon/.tox/py37-pytestmaster/lib/python3.7/site-packages/_pytest/config/__init__.py:758: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: test
   self._mark_plugins_for_rewrite(hook)

Since this is new it would be good to know why it happens now, e.g. by git-bisecting pytest.

[tarpas]

Thanks guys. It's more important to release this than to find out cause of the warning, so it's fine to relax the assertion and create ticket about the warning. I'll have time tomorrow.

[AceFire6]

@blueyed I did a bisect. The process I followed was:

1. Have both pytest and pytest-testmon on my local filesystem
2. Set up a tox config to install the local pytest

   recreate: true
   ...
   localpytest: git+file:///Users/jethromuller/flickswitch/pytest/

3. Create a shell script to run the test in tox during the bisect run

   #!/usr/bin/env bash
   
   cd ~/flickswitch/pytest-testmon/
   tox -e localpytest -- test/test_testmon.py::TestmonDeselect::test_tlf

4. In the pytest directory I did:

   $ git bisect start HEAD 4.5.0
   $ git bisect run ./bisect_test.sh

The result being that the first commit to pass the test was: pytest-dev/pytest@3d01dd3ad

I looked at the pytest files changes in this diff https://github.com/pytest-dev/pytest/compare/3d01dd3ad...master. It seems the removal of this is what might be causing the warning: https://github.com/pytest-dev/pytest/pull/5468/files#diff-1284161c2c98c8aedd66545ff79a8af6L803

I'm not sure if my assessment is correct. But that's what I've found

[tarpas]

@AceFire6 @blueyed thanks a lot guys. Jethro I think you're quite correct with the bisect. I'm not familiar with pytester and assertion rewrites that much, so at first glance I don't know what's the problem. I'm creating a new ticket for this warning (which is emitted from the test inside test), not from our primary test suite.