Write rewritten code using file.write(marshal.dumps())

This works around the fact that some libraries might monkey patch
the file object, so the previous approach of marshal.dump(co, file)
breaks because file is not a built-in file object anymore.

Fix #3503

_pytest/assertion/rewrite.py

@@ -265,10 +265,7 @@ def _write_pyc(state, co, source_stat, pyc):
             mtime = int(source_stat.mtime)
             size = source_stat.size & 0xFFFFFFFF
             fp.write(struct.pack("<ll", mtime, size))
-            if six.PY2:
-                marshal.dump(co, fp.file)
-            else:
-                marshal.dump(co, fp)
+            fp.write(marshal.dumps(co))
     except EnvironmentError as e:
         state.trace("error writing pyc file at %s: errno=%s" % (pyc, e.errno))
         # we ignore any failure to write the cache file

changelog/3503.bugfix.rst

@@ -0,0 +1 @@
+Fix assertion rewriter compatibility with libraries that monkey patch ``file`` objects.