refactor!: Move to src/ layout

[tony]

See also: pypa/packaging.python.org#320

[codecov]

Codecov Report

Merging #356 (6300aa8) into master (1c11885) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   62.26%   62.28%   +0.01%     
==========================================
  Files          88       88              
  Lines        2608     2609       +1     
==========================================
+ Hits         1624     1625       +1     
  Misses        984      984              

Impacted Files	Coverage Δ
src/django_docutils/__about__.py	100.00% <ø> (ø)
src/django_docutils/conftest.py	75.00% <ø> (ø)
src/django_docutils/directives.py	68.96% <ø> (ø)
src/django_docutils/engines.py	41.46% <ø> (ø)
src/django_docutils/exc.py	100.00% <ø> (ø)
...s/favicon/management/commands/prefetch_favicons.py	36.36% <ø> (ø)
src/django_docutils/favicon/models.py	76.47% <ø> (ø)
src/django_docutils/favicon/prefetch.py	75.92% <ø> (ø)
src/django_docutils/favicon/rst/nodes.py	100.00% <ø> (ø)
.../django_docutils/favicon/rst/transforms/favicon.py	56.41% <ø> (ø)
... and 77 more

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