Throw deprecation warnings one level up (out of opaque-keys module)

@cpennington @mmprandom 

Means that in tests, deprecation warnings will look like this:
```python
good_location = SlashSeparatedCourseKey('testOrg', 'testCourse', 'RunBabyRun')
/Users/sarina/edx_all/edx-platform/common/djangoapps/student/tests/test_course_listing.py:98: DeprecationWarning: SlashSeparatedCourseKey is deprecated! Please use locator.CourseLocator
  course_location = SlashSeparatedCourseKey('testOrg', 'doomedCourse', 'RunBabyRun')
```

rather than
```
/Users/sarina/.virtualenvs/edx-platform/src/opaque-keys/opaque_keys/edx/locations.py:44: DeprecationWarning: SlashSeparatedCourseKey is deprecated! Please use locator.CourseLocator
  DeprecationWarning
```