Move to src/ layout

[tony]

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

[codecov-commenter]

Codecov Report

Merging #364 (a1c9535) into master (0cea451) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #364   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines           72        73    +1     
=========================================
+ Hits            72        73    +1     

Impacted Files	Coverage Δ
...lugify_processor/templatetags/slugify_processor.py	100.00% <ø> (ø)
src/django_slugify_processor/text.py	100.00% <ø> (ø)
docs/conf.py	100.00% <100.00%> (ø)

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