add PYTEST_DONT_REWRITE in order to suppress already imported warning

[jonathansp]

After calling pytest.main twice, pytest creates this warning summary

===================================================================== warnings summary =====================================================================
~/lib/python2.7/site-packages/_pytest/assertion/rewrite.py:272
~/lib/python2.7/site-packages/_pytest/assertion/rewrite.py:272: PytestWarning: Module already imported so cannot be rewritten: pytest_benchmark
    self.config,
...

According to

• PYTEST_DONT_REWRITE is not checked for plugins pytest-dev/pytest#2995
• pytest-dev/pytest-cov@22387a6

the doc string PYTEST_DONT_REWRITE can be used to suppress this warning.

[jonathansp]

@ionelmc could you please take a look? thanks

[ionelmc]

Thanks.

[ionelmc]

There will be a release in a week or so if it matters.