CI: Push to codecov with bash script, not python script

[scottclowe]

Having problems pushing with the python script (installed via pip)
and it seems like the big name python packages (numpy, pytest)
tend to use the bash script instead, which is working for them.

I took this more robust version of the download command from pytest https://github.com/pytest-dev/pytest/blob/864338de71d6b56f0029b8ea8f45581001e2bab1/scripts/report-coverage.sh

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@2694532). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #132   +/-   ##
=========================================
  Coverage          ?   82.41%           
=========================================
  Files             ?        9           
  Lines             ?      745           
  Branches          ?      142           
=========================================
  Hits              ?      614           
  Misses            ?       99           
  Partials          ?       32           

Impacted Files	Coverage Δ
fissa/datahandler.py	100.00% <0.00%> (ø)
fissa/deltaf.py	100.00% <0.00%> (ø)
fissa/__meta__.py	100.00% <0.00%> (ø)
fissa/roitools.py	89.07% <0.00%> (ø)
fissa/datahandler_framebyframe.py	100.00% <0.00%> (ø)
fissa/ROI.py	78.72% <0.00%> (ø)
fissa/readimagejrois.py	51.14% <0.00%> (ø)
fissa/neuropil.py	86.84% <0.00%> (ø)
fissa/core.py	94.53% <0.00%> (ø)

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