Update requirements.txt rather than conditionally install pyobjc in the GHA workflow

.github/workflows/test.yml

@@ -43,9 +43,6 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install packages
       run: pip install tox coverage
-    - name: (Optionally install PyObjC package if on macOS platform)
-      if: ${{ runner.os == "macOS" }}
-      run: pip install pyobjc
     - name: Run Tox
       run: tox -e py-cov
     - name: Run Tox without lxml

requirements.txt

@@ -10,3 +10,4 @@ fs==2.4.11
 skia-pathops==0.5.1.post1; platform_python_implementation != "PyPy"
 # this is only required to run Tests/cu2qu/{ufo,cli}_test.py
 ufoLib2==0.6.2
+pyobjc==6.2.2; sys_platform == "darwin"