Fix test_entry_point_create_error (fix #185)

[althonos]

Proposed fix for #185 :

Instead of monkey patching issubclass for it to accept MagicMock as an Opener subclass, I create a mock opener that raises a ValueError on instantiation. Less dirty, and works on Python 3.7.

[coveralls]

Coverage remained the same at 99.976% when pulling 52a6ed4 on althonos:patch-2 into 21cbbfb on PyFilesystem:master.

[althonos]

NB: I also added a version constraint for setuptools in Python 3.3, since agressively updating will cause the build to fail (setuptools 40.0 dropped support for Py3.3).

[willmcgugan]

Thanks @althonos

How would you feel if we dropped support for Py3.3 ?

[althonos]

Depends. For now it does not cost anything to keep support besides installing an older setuptools, so I don't feel we should drop it. We could move it to the allowed failures for the time being though.

[willmcgugan]

I think your right about supporting it so long as its not a major effort.

Thanks for the PR. Is it ready to go?

[willmcgugan]

Thanks

[althonos]

Yes it was 😉