I bumped pytest from 3.5.1 to 3.6.0 ... and the tests fail to run on Windows with a pytest error:

RuntimeError: maximum recursion depth exceeded

If I uninstall 3.6.0 and reinstall 3.5.1 things are back to normal.

See https://ci.appveyor.com/project/nexB/license-expression/build/248/job/smx1qy33w1dxrlh5#L531
and https://ci.appveyor.com/project/nexB/license-expression/build/248/job/jpd8c35tgee9mi7y#L532

Things work fine on POSIX and other Python versions on Windows:
https://travis-ci.org/nexB/license-expression/builds/383421178
https://ci.appveyor.com/project/nexB/license-expression/build/248

The exact versions and wheel of all the thirdparty packages used are here:
https://github.com/nexB/license-expression/tree/fix-symbol-comparison/thirdparty/dev
https://github.com/nexB/license-expression/tree/fix-symbol-comparison/thirdparty/base