style: add module imported but unused in flake8

[oboulant]

Just add the F401 flake8 code to .flake8 so that we have a warning message when there is an unused import.
For instance in src/ruptures/utils/utils.py here with numpy.

[codecov]

Codecov Report

Merging #191 (f23a4c1) into master (d211c5c) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
+ Coverage   96.31%   96.59%   +0.27%     
==========================================
  Files          40       40              
  Lines         978      969       -9     
==========================================
- Hits          942      936       -6     
+ Misses         36       33       -3     

Impacted Files	Coverage Δ
src/ruptures/datasets/pw_linear.py	100.00% <ø> (ø)
src/ruptures/utils/utils.py	93.33% <ø> (+12.38%)	⬆️
src/ruptures/detection/kernelcpd.py	100.00% <100.00%> (ø)
src/ruptures/metrics/hamming.py	100.00% <100.00%> (ø)
src/ruptures/metrics/precisionrecall.py	100.00% <100.00%> (ø)
src/ruptures/utils/__init__.py	100.00% <100.00%> (ø)

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[deepcharles]

Good to me