CI is broken; Python test regarding GeoJSON fails

[JamesLMilner]

One of the python tests fails:

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ERROR: test_create_vector_tiles (tests.test_validate_geojson.TestValidateGeojson)
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Traceback (most recent call last):
  File "/tiler-scripts/tests/test_validate_geojson.py", line 12, in test_create_vector_tiles
    is_valid = validate_geojson(GEOJSON_FILE)
  File "/tiler-scripts/validate_geojson.py", line 11, in validate_geojson
    validation = geojson.is_valid(features)
AttributeError: 'module' object has no attribute 'is_valid'

[JamesLMilner]

Looks like the GeoJSON API has changed. We should probably lock down the Python library versions.