Fix incorrect import of assume

[kochb]

The assume function is incorrectly being imported from hypothesis.strategies instead of hypothesis.

This worked by coincidence in the version of hypothesis available when hypothesis-geojson was released. In modern releases of hypothesis, assume is no longer imported into strategies, causing an ImportError which prevents usage of this library. Since assume was not listed as a member of __all__, the import was inappropriate at the time of release anyway.

Correcting the import to the documented location fixes the ImportError.