drop python3.7 support

[anilbey]

Py3.7's end of life is in 4 months. https://endoflife.date/python

Our NEURON dependency already dropped its support.

[codecov-commenter]

Codecov Report

Merging #446 (4caa494) into master (e87c669) will decrease coverage by 1.24%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
- Coverage   87.91%   86.68%   -1.24%     
==========================================
  Files          50       50              
  Lines        4288     4288              
==========================================
- Hits         3770     3717      -53     
- Misses        518      571      +53     

Impacted Files	Coverage Δ
bluepyopt/ephys/protocols.py	61.37% <0.00%> (-11.56%)	:arrow_down:
bluepyopt/ephys/simulators.py	73.85% <0.00%> (-9.81%)	:arrow_down:
bluepyopt/ephys/parameterscalers/acc_iexpr.py	85.71% <0.00%> (-1.79%)	:arrow_down:
bluepyopt/deapext/utils.py	95.31% <0.00%> (-1.57%)	:arrow_down:
bluepyopt/ephys/stimuli.py	87.34% <0.00%> (-1.27%)	:arrow_down:
bluepyopt/ephys/locations.py	86.47% <0.00%> (-0.49%)	:arrow_down:

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