Use the pre-commit package with flake8 and black

[mwouts]

Todo:

• Run flake8 before and after black, in case black breaks something (cf. the first run documented at Activate pre-commit reformatting and checks for the Jupytext repository #482) EDIT: this is not necessary, as the commit is postponed when black changes a file.
• Document how to install the pre-commit package in CONTRIBUTING.md

Closes #482

[codecov]

Codecov Report

Merging #483 into master will increase coverage by 0.00%.
The diff coverage is 99.02%.

@@           Coverage Diff           @@
##           master     #483   +/-   ##
=======================================
  Coverage   98.75%   98.76%           
=======================================
  Files          81       81           
  Lines        8060     8067    +7     
=======================================
+ Hits         7960     7967    +7     
  Misses        100      100           

Impacted Files	Coverage Δ
tests/test_read_simple_rmd.py	100.00% <ø> (ø)
tests/test_read_simple_rust.py	100.00% <ø> (ø)
tests/test_read_simple_scheme.py	100.00% <ø> (ø)
tests/test_read_simple_sphinx.py	100.00% <ø> (ø)
tests/test_read_write_functions.py	100.00% <ø> (ø)
tests/test_rmd_to_ipynb.py	100.00% <ø> (ø)
tests/test_save_multiple.py	100.00% <ø> (ø)
tests/test_stringparser.py	100.00% <ø> (ø)
tests/test_trust_notebook.py	100.00% <ø> (ø)
tests/test_unicode.py	100.00% <ø> (ø)
... and 80 more

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