fix coverage for internal functions (#280)

the mock wrapper for internal functions would report a file of
`"VyperContract"`. we can ignore these (as soon as they enter into the
actual module, the pc traces should be correct anyway).

boa/coverage.py

@@ -60,7 +60,10 @@ def _valid_frame(self, frame):
     def dynamic_source_filename(self, filename, frame):
         if not self._valid_frame(frame):
             return None
-        return frame.f_locals["filename"]
+        ret = frame.f_locals["filename"]
+        if ret is not None and not ret.endswith(".vy"):
+            return None
+        return ret
 
     def has_dynamic_source_filename(self):
         return True