Remove the dependency on mock

[mwouts]

In Python 3, mock is available in base Python as unittest.mock. See #279

[codecov]

Codecov Report

Merging #285 into master will decrease coverage by 1.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #285     +/-   ##
=========================================
- Coverage    99.2%   98.01%   -1.2%     
=========================================
  Files          68       69      +1     
  Lines        6667     6644     -23     
=========================================
- Hits         6614     6512    -102     
- Misses         53      132     +79

Impacted Files	Coverage Δ
tests/test_cli.py	97.73% <100%> (-2.27%)	⬇️
tests/test_escape_magics.py	100% <100%> (ø)	⬆️
jupytext/contentsmanager.py	98.45% <100%> (ø)	⬆️
tests/test_header.py	100% <100%> (ø)	⬆️
tests/test_active_cells.py	100% <100%> (ø)	⬆️
tests/test_read_simple_markdown.py	100% <100%> (ø)	⬆️
tests/test_load_multiple.py	100% <100%> (ø)	⬆️
tests/test_trust_notebook.py	100% <100%> (ø)	⬆️
tests/test_rmd_to_ipynb.py	100% <100%> (ø)	⬆️
tests/test_mirror.py	98.96% <100%> (-1.04%)	⬇️
... and 10 more

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