ci.yml: use Python virtual environment for macos14

because I was getting such an error:

× This environment is externally managed
╰─> To install Python packages system-wide, try brew install
    xyz, where xyz is the package you are trying to
    install.

    If you wish to install a non-brew-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip.

.github/workflows/ci.yml

@@ -10,16 +10,18 @@ jobs:
     env:
       SKBUILD_CMAKE_ARGS: "-DEXTRA_WARNINGS=ON;-DFETCH_ZLIB_NG=ON"
       SKBUILD_CMAKE_TARGETS: "all;check"
-      PIP_REQUIRE_VIRTUALENV: "false"
     steps:
     - uses: actions/checkout@v4
     - name: Build wheel and install
       run: |
+        python3 -m venv pyenv
+        source pyenv/bin/activate
         python3 -m pip install wheel
         python3 -m pip wheel -v .
         python3 -m pip install gemmi-*.whl
     - name: Python Tests
       run: |
+        source pyenv/bin/activate
         python3 -m unittest discover -v -s tests/
         python3 -m pip install sphinx
         cd docs