include opencv-python in instructions to generate SDP requirements

[zacharyburnett]

Resolves JP-3109

spacetelescope/stcal#126 and spacetelescope/stcal#136 made opencv-python an optional dependency (as built binaries are not available on all user platforms). This PR adds opencv-python to the instructions for creating requirements-sdp.txt, to ensure the environment has opencv-python installed.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[zacharyburnett]

Options going forward:

1. restore opencv-python to a hard dependency in setup.cfg - this would break some users' workflows, but perhaps they can move to a platform that has built binaries, or figure out how to build opencv-python on their own
2. use a less heavy dependency
   • scikit-image and shapely can perform similar functionality; however, the resulting snowball masks are slightly different replace opencv-python with scikit-image for snowball detection stcal#138
   • Alternatively, we could use astropy regions:

     Astropy regions can do this easily. You can make ellipses and circles and then export them as pixel masks. And also interface with shapely if needed.
     Opencv dependency in stcal being optional causes uncaught error in jump step #7409 (comment)

3. use binary operations instead of ellipse / circle fitting:

   Perhaps going too far, a much simpler approach would be to not fit circles and ellipses at all (which can lead to fitting errors and masking half the image), but to use scipy.ndimage.binary_dilation instead. Just grow regions. And select regions of a minimum area to grow using scipy.ndimage.binary_erosion. This is vastly simpler and probably more robust. And handles shapes that are not exactly circles or ellipses.
   Opencv dependency in stcal being optional causes uncaught error in jump step #7409 (comment)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4285c4e) 77.58% compared to head (c2ff3af) 77.58%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7474   +/-   ##
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  Coverage   77.58%   77.58%           
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  Files         449      449           
  Lines       36029    36029           
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  Hits        27952    27952           
  Misses       8077     8077           

Flag	Coverage Δ		*Carryforward flag
nightly	77.58% <ø> (ø)		Carriedforward from 4285c4e
unit	49.57% <ø> (ø)

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