Fixed #18409 -- Made RegexField work with unicode characters.

Resolves Flatpages test issues by:
 - Creating an example_site fixture
 - Overriding project SITE_ID setting to 1
 - Normalizing the use of the hardcoded (1) site_id to settings.SITE_ID

When generating error message in to_python, any unicode string
containing non-ascii characters triggered a UnicodeEncodeError for
most field types.

gzip.GzipFile does not support files with mode set to None.

Fixed #18393 -- Prevented blocktrans to crash when a variable name is badly formatted.

Now that call_command does not raise SystemExit any more, we can
use call_command again for testing compilemessages.

Raising CommandError whenever a management command meets an error
condition is the standard way to handle errors in commands.

Moved sys.exit(1) so as failing management commands reach it
only when running from command line.

On MySQL when checking the server version, a new connection could be
created but never closed. This could result in open connections on
server startup.

Generic cleanup and dead code removal in deferred model field loading
and model.__reduce__().

Also fixed an issue where if an inherited model with a parent field
chain parent_ptr_id -> id would be deferred loaded, then accessing
the id field caused caused a database query, even if the id field's
value is already loaded in the parent_ptr_id field.

test_abort_loaddata_on_error was exactly the same test as
test_empty (error is tested in test_error_message).

This should prevent storing all rows in memory when big sets of
data are dumped.
See ticket for heroic contributors.

Thanks rmattb for the report and the patch.

It is now possible to set WMS options by overriding wms_options
in a subclass of GeoModelAdmin.
Thanks slinkp for the report and the initial patch.

Thanks Vinaj Sajip for the help of his django3 branch.

The OPTIONS, PUT and DELETE methods no longer apply arbitrary
data encoding (in the query string or in the request body).

When order_by causes new joins to be added to the query, the joins must
be LEFT OUTER joins for nullable relations, otherwise the order_by
could cause the results to be altered. This commit fixes the logic to
only promote new joins, previously all joins in the order_by lookup
path were promoted.

Thanks to Bruno Desthuilliers for spotting this corner case.

Test suite is passing, but if any regression would be detected,
we might replace smart_str by force_unicode.

This was recently fixed for one-to-one relations; this patch adds
support for foreign keys. Thanks kaiser.yann for the report and
the initial version of the patch.

Thanks geoffhing@gmail.com for the report.

In particular, make tests independent of each other.

In Python 3, types.NoneType is no more available.

Added a new feature to allow 3rd party backends to skip tests which
test sequence resetting.

Thanks to manfre for report and patch.

Thanks to manfre for report and patch.

A test in model_fields used LEN() in raw SQL. This function is not
available on some 3rd party backends. I removed this function and
ensured that the test works correctly (breaks pre e9bbdb39) with
the change.

The new construct is also Python 3 compatible (where 'next' has
been renamed to '__next__').