Fixed #23663 -- Initialize output streams for BaseCommand in __init__().

This helps with testability of management commands.

Thanks to trac username daveoncode for the report and to
Tim Graham and Claude Paroz for the reviews.
Made testing of stdout and stderr more consistent.

Refs #23663.
Removed unused import from the JSON serialization example.

This is a leftover from 5612f54bd56086e2a375e86474ec734c172e7d1f.
Fixed #23686 -- Tweak color output of the system check framework.

Thanks Tim Graham for the review.
Refactored color_style() and no_style() to improve testability. Refs #23663.

This includes the following improvements:

- The type of the style object is now called 'Style' rather than 'dummy'.
- The new make_style() function allows generating a Style object directly
  from a config string. Before the only way to get a style object was
  through the environ and it also required that the terminal supported
  colors which isn't necessarily the case when testing.
- The output of no_style() is now cached with @lru_cache.
- The output of no_style() now has the same set of attributes as the
  other Style objects. Previously it allowed anything to pass through
  with __getattr__.
Added release dates to release notes.
Fixed #23701 -- Removed an unneeded check in collectstatic.

Thanks prathik for the report.
Added stub release notes for 1.7.2.
Fixed #23621 -- Warn for duplicate models when a module is reloaded.

Previously a RuntimeError was raised every time two models clashed
in the app registry. This prevented reloading a module in a REPL;
while it's not recommended to do so, we decided not to forbid this
use-case by turning the error into a warning.

Thanks @dfunckt and Sergey Pashinin for the initial patches.
Improved warning message when reloading models. Refs #23621.

Thanks dfunckt and Tim Graham.
Allowed test from refs #23612 to be run from a directory other than tests.