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

CHANGELOG.rst

@@ -18,7 +18,10 @@
 * Fix error message using ``approx`` with complex numbers (`#2082`_).
   Thanks `@adler-j`_ for the report and `@nicoddemus`_ for the PR.
 
-*
+* Remove internal code meant to support earlier Python 3 versions that produced the side effect
+  of leaving ``None`` in ``sys.modules`` when expressions were evaluated by pytest (for example passing a condition
+  as a string to ``pytest.mark.skipif``)(`#2103`_).
+  Thanks `@jaraco`_ for the report and `@nicoddemus`_ for the PR.
 
 * Cope gracefully with a .pyc file with no matching .py file (`#2038`_). Thanks
   `@nedbat`_.
@@ -41,6 +44,7 @@
 .. _#2038: https://github.com/pytest-dev/pytest/issues/2038
 .. _#2078: https://github.com/pytest-dev/pytest/issues/2078
 .. _#2082: https://github.com/pytest-dev/pytest/issues/2082
+.. _#2103: https://github.com/pytest-dev/pytest/issues/2103
 
 
 3.0.4

_pytest/_code/source.py

@@ -192,14 +192,6 @@ def compile(self, filename=None, mode='exec',
             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