Add environment-ros3.yml to MANIFEST.in

[rly]

Motivation

environment-ros3.yml was added in PyNWB 2.0 but not added to MANIFEST.in. It should be added so that it can be included in source distributions alongside the requirements files.

This file is currently used only by CI processes, but conda users may wish to use it to set up a working ROS3-compatible conda environment.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1398 (5a1a150) into dev (a27200d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1398   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          37       37           
  Lines        2714     2714           
  Branches      450      450           
=======================================
  Hits         2097     2097           
  Misses        535      535           
  Partials       82       82           

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