Make pyproj a runtime dependency

[maximlt]

#583 added a direct import from pyproj, while before the code was using it only when it was already imported (looking intosys.modules). Since cartopy depends on pyproj, and GeoViews depends on cartopy, it seems fine to add pyproj as a runtime dependency to GeoViews.

[hoxbro]

It was not a deliberate decision to add it as a runtime dependency on my part.

But I agree with adding it as a runtime dependency.