Bump version to v1.7.3
Merge pull request #1499 from lbdreyer/tag_v1.7.3

Bump version to v1.7.3
BUG: Handling of guess_bounds for circular coords
MAINT: Removed convoluted test.
MAINT: Review changes
MAINT: Part migration of nearest neighbour extraction to unit tests
SSL certificate verification workaround.
BUG: Fixed nn index for descending bounds

Fixes a bug in Coord.nearest_neighbour_index for handling coordinates
with descending bounds.