Continuous integration: resolve docutils installation step build failures (#11331)

* continuous integration: add comment explaining PYTHONWARNINGS env var to main unit test workflow

* continuous integration: unit test workflow: report-but-do-not-fail
  when warnings about pkg_resources deprecation are encountered during
  docutils dependency installation

* continuous integration: unit test workflow: instead of reducing the
  severity of all DeprecationWarnings during docutils (non-dev)
  installation, apply two message filters - one each for the warnings
  encountered so far

* continuous integration: unit test workflow: add 'module' and 'line'
  fields for warning filters

Ref:
https://docs.python.org/3/library/warnings.html#describing-warning-filters


---------

Co-authored-by: Jean-François B <2589111+jfbu@users.noreply.github.com>

.github/workflows/main.yml

@@ -13,7 +13,7 @@ env:
   FORCE_COLOR: "1"
   PYTHONDEVMODE: "1"  # -X dev
   PYTHONWARNDEFAULTENCODING: "1"  # -X warn_default_encoding
-  PYTHONWARNINGS: "error,always:unclosed:ResourceWarning"
+  PYTHONWARNINGS: "error,always:unclosed:ResourceWarning::"  # default: all warnings as errors, except ResourceWarnings about unclosed items
 
 jobs:
   ubuntu:
@@ -59,6 +59,8 @@ jobs:
     - name: Install Docutils ${{ matrix.docutils }}
       run: python -m pip install --upgrade "docutils==${{ matrix.docutils }}.*"
       if: "!endsWith(matrix.python, '-dev')"
+      env:
+        PYTHONWARNINGS: "error,default:pkg_resources is deprecated:DeprecationWarning::,default:Deprecated call to `pkg_resources.declare_namespace:DeprecationWarning::"
     - name: Install Docutils ${{ matrix.docutils }} (ignore warnings)
       run: python -m pip install --upgrade "docutils==${{ matrix.docutils }}.*"
       if: "endsWith(matrix.python, '-dev')"