GeoDjango spatial reference fixes

[mjakob]

I have made things!

• Make django.contrib.gis.gdal.srs.SpatialReference() accept an EPSG code as an int, not just a string.
• Make django.contrib.gis.gdal.srs.CoordTransform() only accept SpatialReferences as arguments.
• Narrow django.contrib.gis.geos.GEOSGeometryBase.transform() and django.contrib.gis.gdal.OGRGeometry.transform() to accept CoordTransform, SpatialReference, int, or str instead of Any.

All changes should be according to Django's documentation. A test case is included.

Related issues

None that I am aware of.

[sobolevn]

I can say honestly, that I don't have any clue how any of this works :)

Maybe some other maintainers have experience with geo module?

[sobolevn]

Thanks, but I don't think this is worth adding a new test case.

We add tests for plugin related things / complex overloads.

[mjakob]

Removed!

I hesitated myself after I wrote it, but when I saw gis_tests in the ignored test modules i decided to keep it in.

[mjakob]

If it's of any help, the Django code is pretty clear with what types it expects:

• CoordTransform()
• OGRGeometry.transform()
• GEOSGeometryBase.transform() actually creates an OGRGeometry from the GEOSGeometry and calls that instance's transform() method.

Finally, the Django documentation has an example of creating a SpatialReference from an integer. This is what triggered this PR since it causes an error in mypy.

[intgr]

Sorry that this got delayed for so long. None of the regular TypedDjango reviewers are familiar with GeoDango. But this looks good to me.