Rtree

- RTree wheel building infrastructure 163. Thanks to
Michael Dawson-Haggerty mikedh for a 134 commit marathon to construct automated wheel building of libspatialindex and Rtree.

- Add index size and custom `__repr__` methods for index classes 139 (thanks sr-murthy )
- nearest picking just first item 140
- CI not using conda-forge builds 142
- Drop Python 2 support 137 (thanks sr-murthy)
- Flake8 linting 136 (thanks sr-murthy)

- find_library and libspatialindex library loading https://github.com/Toblerity/rtree/pull/131

- Refactored tests to be based on unittest https://github.com/Toblerity/rtree/pull/129
- Update libspatialindex library loading code to adapt previous behavior https://github.com/Toblerity/rtree/pull/128
- Empty data streams throw exceptions and do not partially construct indexes https://github.com/Toblerity/rtree/pull/127

0.9.1 is a patch release to fix library finding of libspatialindex.

* Fix libspatialindex_c.so finding https://github.com/Toblerity/rtree/issues/120

Enhancements
- Add Index.GetResultSetOffset()
- Add Index.contains() method for object and id (requires libspatialindex 1.9.3+) 116
- Add Index.Flush() 107
- Add TPRTree index support (thanks sdhiscocks 117 )
- Return container sizes without returning objects 90
- Index.get_results_size and Index.get_results_index for index paging support

Bug fixes
- Better exceptions in cases where stream functions throw 80
- CI migrated to Azure Pipelines https://dev.azure.com/hobuinc/rtree/_build?definitionId=5
- Test enhancements. libspatialindex 1.8.5 and 1.9.3 now tested with CI