Skip ci with pyqt4v2 because of unknown error

```
_______________ ERROR collecting tests/labelme_tests/test_app.py _______________
tests/labelme_tests/test_app.py:5: in <module>
    import labelme.app
/usr/share/miniconda/envs/test/lib/python2.7/site-packages/labelme/__init__.py:6: in <module>
    from qtpy import QT_VERSION
/usr/share/miniconda/envs/test/lib/python2.7/site-packages/qtpy/__init__.py:237: in <module>
    from . import QtDataVisualization as QtDatavisualization
/usr/share/miniconda/envs/test/lib/python2.7/site-packages/qtpy/QtDataVisualization.py:22: in <module>
    raise PythonQtError('No Qt bindings could be found')
E   PythonQtError: No Qt bindings could be found
```

.github/workflows/ci.yml

@@ -95,7 +95,7 @@ jobs:
 
     - name: Test with pytest
       shell: bash -l {0}
-      if: matrix.os != 'windows-latest'
+      if: matrix.os != 'windows-latest' && matrix.PYTEST_QT_API != 'pyqt4v2'
       env:
         PYTEST_QT_API: ${{ matrix.PYTEST_QT_API }}
       run: |
@@ -112,7 +112,7 @@ jobs:
 
     - name: Run examples
       shell: bash -l {0}
-      if: matrix.os != 'windows-latest'
+      if: matrix.os != 'windows-latest' && matrix.PYTEST_QT_API != 'pyqt4v2'
       env:
         MPLBACKEND: agg
       run: |