Unbind pytest dep to fix running hypothesis tests

Forcing very old version of pytest has resulted in hypothesis tests
being implicitly skipped:

```
/tmp/brotlicffi/.tox/py38/lib/python3.8/site-packages/hypothesis/extra/pytestplugin.py:60: HypothesisWarning:
        You are using Pytest version 3.8.1.  Hypothesis tests work with any test
        runner, but our Pytest plugin requires Pytest  4.3 or newer.
        Note that the Pytest developers no longer support this version either!
        Disabling the Hypothesis pytest plugin...
```

Let tests use the newest version of pytest to fix that.  While at it,
we also need to allow newer pytest-cov.

Author: mgorny

test_requirements.txt

@@ -1,3 +1,3 @@
-pytest==3.8.1
-pytest-cov==2.6.0
+pytest
+pytest-cov
 hypothesis