Issue #110: Restrict black pin to py>=3.9 and run linting once

Follow-up to the previous commit. Pinning black==25.1.0 unconditionally broke
the Python 3.8 matrix job at install time:

  ERROR: Could not find a version that satisfies the requirement
  black==25.1.0; extra == "dev" ... No matching distribution found

black 25.1.0 requires Python >=3.9. Fail-fast then cancelled the other 12 jobs,
which is why they showed as failures despite only one real error.

Two changes:

1. Marker the pin as `black==25.1.0; python_version >= '3.9'` so the 3.8 job
   installs cleanly without black.

2. Gate the black and flake8 steps on ubuntu-latest/3.11. Linting is
   version-independent, so running it on all 13 matrix combinations wastes CI
   and, more importantly, made the formatter a function of the matrix entry.
   This follows the convention already used by the coverage-badge steps in
   this workflow.

Note for #113: the flake8 step passes --exit-zero, so it cannot fail and has
never gated anything. Left as-is here because removing it would surface a
large backlog and block this P0; recorded on #113 instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012gTBDPK16HUZ3kHQ2QyjuU

Author: jeremymanning

.github/workflows/tests.yml

@@ -38,11 +38,16 @@ jobs:
         python -m pip install --upgrade pip
         pip install -e ".[dev,test,kubernetes,widget]"
 
+    # Linting is version-independent, so run it once on the primary target
+    # rather than on all 13 matrix combinations. This also keeps the black pin
+    # off Python 3.8, which black 25.1.0 does not support (see #110).
     - name: Lint with black
+      if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
       run: |
         black --check clustrix/ tests/
 
     - name: Lint with flake8
+      if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'
       run: |
         flake8 clustrix/ tests/ --max-line-length=88 --extend-ignore=E203,W503,F401,E722,F541,F841,F811,E731,E501,W291,W293,F824 --exit-zero
 

pyproject.toml

@@ -83,7 +83,7 @@ cloud = [
 dev = [
     "pytest>=6.0",
     "pytest-cov>=2.0",
-    "black==25.1.0",  # pinned - unbounded ">=" let CI pull black 26.5.1 (see #110)
+    "black==25.1.0; python_version >= '3.9'",  # pinned - unbounded ">=" let CI pull 26.5.1 (see #110)
     "flake8>=3.8", 
     "mypy>=0.812",
     "types-PyYAML",
@@ -132,7 +132,7 @@ all = [
     # Development dependencies
     "pytest>=6.0",
     "pytest-cov>=2.0",
-    "black==25.1.0",  # pinned - unbounded ">=" let CI pull black 26.5.1 (see #110)
+    "black==25.1.0; python_version >= '3.9'",  # pinned - unbounded ">=" let CI pull 26.5.1 (see #110)
     "flake8>=3.8",
     "mypy>=0.812",
     # Documentation dependencies