Create tests for PythonExpressionsMonitor

[rennerocha]

This feature is not tested: https://github.com/scrapinghub/spidermon/blob/master/spidermon/python/factory.py#L12

[rennerocha]

#89 happened because the lack of tests of this class.

[leovictorsr]

@rennerocha I started to work on this issue and I have a simple question. As a factory of python expression monitors, the tests for this feature should be under the test_monitors class, or it would be good to have it in a different class for clarity (as test_monitors tests refer to built-in scrapy monitors).

Also, could you give me a simple example of expression we are expected to receive to be monitored?

[rosheen33]

https://github.com/scrapinghub/spidermon/blob/master/tests/contrib/scrapy/test_extensions.py

[mirceachira]

Is this still relevant&available? I'd like to work this if possible