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

[cimarronm]

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()

[cimarronm]

@jenshnielsen, can you review to make sure it looks fine or needs further modifications

[jenshnielsen]

It looks sensible to me, I don't know why I didn't see this and Travis is happy without it.

The build is currently failing on Travis. I think this is because you need to instantiate mocksip before using it. I made a pr against your branch to fix this.

[jenshnielsen]

Travis is happy. And the change looks good to me. Merging.