Shorten the name of variable "python-version"

Using "python" shortens some lines and is clear enough.

Author: mhucka

.github/workflows/nightly-tests.yaml

@@ -40,7 +40,7 @@ concurrency:
 jobs:
   Pytest:
     # Try to fit as much info as possible into the GHA sidebar at run-time.
-    name: Py ${{matrix.python-version}} + ${{matrix.os}}/${{matrix.arch}} + ${{matrix.cirq}}
+    name: Py ${{matrix.python}} + ${{matrix.os}}/${{matrix.arch}} + ${{matrix.cirq}}
     runs-on: ${{matrix.os}}
     timeout-minutes: 15
     strategy:
@@ -49,7 +49,7 @@ jobs:
         # These Python versions were carefully selected by trial and error
         # to be available on as many os/arch combos as possible, while also
         # making all necessary Python dependencies available on those combos.
-        python-version: ['3.11.9', '3.12.7', '3.13.1']
+        python: ['3.11.9', '3.12.7', '3.13.1']
         os: [ubuntu-24.04, ubuntu-24.04-arm, macos-15, macos-14, windows-2025]
         cirq: ['cirq-core', 'cirq-core~=1.0.dev0']
         arch: [x64, arm64]
@@ -76,11 +76,11 @@ jobs:
       - name: Check out a copy of the OpenFermion git repository
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
 
-      - name: Set up Python ${{matrix.python-version}}
+      - name: Set up Python ${{matrix.python}}
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
         id: cache
         with:
-          python-version: ${{matrix.python-version}}
+          python: ${{matrix.python}}
           architecture: ${{matrix.arch}}
           cache: pip
           cache-dependency-path: dev_tools/requirements/envs/pytest.env.txt