Remove obsolete use of OrderedDict with Python 3.6+

[Nick Pope]

With Django 3.0 supporting Python 3.6+ we can strip out some of the use of OrderedDict that is no longer required now that dictionaries preserve insertion order. Although this property of dictionaries is only a language feature from Python 3.7, it is unlikely to be changed in a patch release of CPython 3.6 and PyPy3 already preserves insertion order.

[Nick Pope]

​PR

[Tim Graham <timograham@…>]

In 24b82cd2:

Fixed #30159 -- Removed unneeded use of OrderedDict.

Dicts preserve order since Python 3.6.

[Jacob Walls <jacobtylerwalls@…>]

In 71426673:

Replaced yaml serializer workaround to preserve dict order with PyYAML's sort_keys=False.

The sort_keys parameter was added in PyYAML 5.1 (released March 2019).
The workaround was added in 24b82cd201e21060fbc02117dc16d1702877a1f3
(refs #30159).

[Mariusz Felisiak <felisiak.mariusz@…>]

In d33364c7:

Removed OrderedDict representer from PyYAML serializer.

Added in 5bc3123479bd97dc9d8a36fa9a3421a71063d1da (refs #24558),
it was obsoleted when OrderedDict usage was removed in
24b82cd201e21060fbc02117dc16d1702877a1f3 (refs #30159).