Remove support code for earlier Python 3 version in Source.compile

This code leaves None in sys.modules as a side effect but is no longer needed in the Python 3 versions we support.

Fix #2103

_pytest/_code/source.py

@@ -192,14 +192,6 @@ class Source(object):
             if flag & _AST_FLAG:
                 return co
             lines = [(x + "\n") for x in self.lines]
-            if sys.version_info[0] >= 3:
-                # XXX py3's inspect.getsourcefile() checks for a module
-                # and a pep302 __loader__ ... we don't have a module
-                # at code compile-time so we need to fake it here
-                m = ModuleType("_pycodecompile_pseudo_module")
-                py.std.inspect.modulesbyfile[filename] = None
-                py.std.sys.modules[None] = m
-                m.__loader__ = 1
             py.std.linecache.cache[filename] = (1, None, lines, filename)
             return co