Update to doc/conf.py to allow for building docs without qt installed

Had to modify `mocksip` in `conf.py` from a simple MagicMock object to one with a `getapi` function to avoid the following type error:
```
Exception occurred:
  File "/Users/cimarron/github/matplotlib/lib/matplotlib/backends/qt_compat.py", line 94, in <module>
    if sip.getapi("QString") > 1:
TypeError: unorderable types: MagicMock() > int()
```