create(no_annex=False) -> create(annex=True)

[mih]

This avoids double-negative thinking when using the Python API, and fixes gh-4207

There is no change wrt the command-line API.

A warning is emitted if, and only if, using the Python API no_annex=True is given in conflict with the value of annex.

In order to make this a smooth transition (but possibly with further future benefits), it is now possible to have Python-only arguments for our commands.

This can be achieved by setting the args of a Parameter instance to be an empty tuple. The value None continues to indicate that a default argument name should be derived from the
parameter name. Example:

some_param=Parameter(args=tuple(), ...)

[codecov]

Codecov Report

Merging #4321 into master will increase coverage by 0.04%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #4321      +/-   ##
==========================================
+ Coverage   88.83%   88.88%   +0.04%     
==========================================
  Files         285      285              
  Lines       37457    37471      +14     
==========================================
+ Hits        33275    33306      +31     
+ Misses       4182     4165      -17     

Impacted Files	Coverage Δ
datalad/core/local/create.py	93.00% <71.42%> (-1.12%)	⬇️
datalad/core/distributed/tests/test_push.py	99.62% <100.00%> (ø)
datalad/core/local/tests/test_create.py	100.00% <100.00%> (ø)
datalad/core/local/tests/test_save.py	96.75% <100.00%> (ø)
...lad/distribution/tests/test_create_test_dataset.py	100.00% <100.00%> (ø)
datalad/distribution/tests/test_dataset.py	99.66% <100.00%> (ø)
datalad/distribution/tests/test_get.py	100.00% <100.00%> (ø)
datalad/distribution/tests/test_install.py	99.55% <100.00%> (ø)
datalad/distribution/tests/test_update.py	97.72% <100.00%> (ø)
datalad/interface/base.py	90.90% <100.00%> (+0.05%)	⬆️
... and 11 more

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