Django-gisserver

* CRS parsing no longer raises SyntaxErrors.

* Fixed type assertion when `django.contrib.postgres` was not installed.

* Added "geojson" as output format alias in `GetCapabilities` for ESRI ArcGIS online.

* Optimized response writing, buffering provement gave ~12-15% speedup.
* Optimized GML response, reduced response size by ~9% by removing whitespace.

* Fixed regression for auto-correcting xmlns for `<Filter>` tags that have leading whitespace.
* Fixed weird crashes when geometry field is not provided.
* Simplify `FeatureType.geometry_field` logic.

* Added support for `maxOccurs=unbounded` elements (M2M, reverse foreign key, array fields).
* Added support for filtering on M2M, reverse foreign key and array fields.
* Added `field(..., xsd_class=...)` parameter to simplify overriding `FeatureField.xsd_element_class`.
* Added `xsd_base_type` parameter to `ComplexFeatureField` to allow overriding it.
* Added `GISSERVER_DB_PRECISION` setting and made precision consistent between output formats.
* Added `.prefetch_related()` support to large GeoJSON responses (using prefetching on iterator chunks).
* Added `FeatureType.filter_related_queryset()` that allows adjusting all retrieved querysets.
* Using `.only()` to reduce transferred data to the actual fields.
* Optimized large responses through the chunked-iterator and `.only()` usage (seen 2x improvement on GeoJSON on some large datasets).
* Improved `<fes:Filter>` error handling, avoid internal server errors for missing XML child elements.
* Improved GeoJSON content type in HTTP responses, using `application/geo+json` instead of `application/json`.
* Fixed next/previous pagination links, to preserve lowercase querystring fields that project-specific code might use.
* Fixed preserving extra querystring fields in HTML pages.
* Fixed integration with custom fields, using `value_from_object()` instead of `getattr()` to retrieve field values.
* Fixed calculating extent over feature with mixed geometry types.
* Various code cleanups.
* Dropped Python 3.6 support.