Remove dependency on testfixtures

[mwouts]

#279

[codecov]

Codecov Report

Merging #283 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
- Coverage    99.2%   99.18%   -0.02%     
==========================================
  Files          68       68              
  Lines        6625     6500     -125     
==========================================
- Hits         6572     6447     -125     
  Misses         53       53

Impacted Files	Coverage Δ
tests/test_cli.py	100% <100%> (ø)	⬆️
tests/test_read_simple_rmd.py	100% <100%> (ø)	⬆️
tests/test_escape_magics.py	100% <100%> (ø)	⬆️
tests/test_black.py	100% <100%> (ø)	⬆️
tests/test_read_simple_markdown.py	100% <100%> (ø)	⬆️
tests/test_header.py	100% <100%> (ø)	⬆️
tests/test_active_cells.py	100% <100%> (ø)	⬆️
tests/test_read_simple_julia.py	100% <100%> (ø)	⬆️
tests/test_read_simple_clojure.py	100% <100%> (ø)	⬆️
tests/test_pep8.py	98.36% <100%> (-0.03%)	⬇️
... and 45 more

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