Gtfs-kit

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- Added support for Python 3.9 and dropped support for Python 3.6.

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- Specified in more detail the Rtree dependency.

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- Bugfix: properly set the ``use_utm`` flag in ``build_geometry_by_shape()`` and ``build_geometry_by_stop()``.

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- Breaking change: refactored ``get_stops_in_polygon()`` to ``get_stops_in_area()``, which accepts a GeoDataFrame.
- Breaking change: refactored ``restrict_to_polygon()`` to ``restrict_to_area()``, which accepts a GeoDataFrame.
- Breaking changes: refactored ``compute_center()`` to ``compute_centroid()``.
- Updated ``get_utm_crs()`` to differentiate between northern and southern hemispheres.
- Added more defensive copying after subsetting some DataFrames.
- Fixed calendar_dates table in ``restrict_to_dates()``.
- Added ``compute_convex_hull()`` to Feed methods. Forgot about that function.
- Switched from using route IDs to using route short names for layer names in ``map_routes()``.

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- Fixed a CRS deprecation warning as requested in `Pull Request 5 <https://github.com/mrcagney/gtfs_kit/pull/5>`_.
- Changed ``get_utm_crs()`` to output an EPSG CRS string, e.g. "EPSG:32655", instead of a PROJ4 definition string. Did this under the recommendation of the `GeoPandas docs <https://geopandas.org/projections.html#manually-specifying-the-crs>`_.
- Fixed CRS mismatch warning in ``compute_screen_line_counts()``.
- Updated dependencies and included Python 3.8 support.

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- Bugfix: got ``read_feed()`` working on Windows thanks to `Pull Request 4 <https://github.com/mrcagney/gtfs_kit/pull/4>`_.