Ensure rewritten modules don't inherit __future__ flags from pytest

In a recent refactoring we enabled all __future__ features in pytest
modules, but that has the unwanted side effect of propagating those
features to compile()'d modules inside assertion rewriting, unless
we pass dont_inherit=False to compile().

_pytest/assertion/rewrite.py

@@ -337,7 +337,7 @@ def _rewrite_test(config, fn):
         return None, None
     rewrite_asserts(tree, fn, config)
     try:
-        co = compile(tree, fn.strpath, "exec")
+        co = compile(tree, fn.strpath, "exec", dont_inherit=True)
     except SyntaxError:
         # It's possible that this error is from some bug in the
         # assertion rewriting, but I don't know of a fast way to tell.

testing/test_assertrewrite.py

@@ -712,6 +712,24 @@ def test():
         result.stdout.fnmatch_lines(['*= 1 passed in *=*'])
         assert 'pytest-warning summary' not in result.stdout.str()
 
+    @pytest.mark.skipif(sys.version_info[0] > 2, reason='python 2 only')
+    def test_rewrite_future_imports(self, testdir):
+        """Test that rewritten modules don't inherit the __future__ flags
+        from the assertrewrite module.
+
+        assertion.rewrite imports __future__.division (and others), so
+        ensure rewritten modules don't inherit those flags.
+
+        The test below will fail if __future__.division is enabled
+        """
+        testdir.makepyfile('''
+            def test():
+                x = 1 / 2
+                assert type(x) is int
+        ''')
+        result = testdir.runpytest()
+        assert result.ret == 0
+
 
 class TestAssertionRewriteHookDetails(object):
     def test_loader_is_package_false_for_module(self, testdir):