Improve lookup/request to allow access to class instances from staticfunctions

[nicoddemus]

Follow up from #3705 (comment)

We would like to cleanup the __being_called_by_pytest keyword hack that was used in #3705. @RonnyPfannschmidt's suggestion of unwrapping the fixture function just before the call works great, except that for staticmethods we don't have access to instance request at that point.

It seems like a deeper issue in the structural construction of the request: static test functions should still have self in request.instance.

[pytestbot]

GitMate.io thinks possibly related issues are #3359 (Allow methods to access a class fixture. ), #2138 (Accessing test instance object from pytest_runtest_makereport), #765 (Re-allow simple class::test selection via "-k"), #2270 (Question: writing a plugin with a fixture that can access its plugin instance properties), and #674 (Fixtures from methods wrongly get rebound to request.instance).