use pyqt6 instead of pyqt5

PyQt5 causes installation issues on M1 macs (see issue #83).
PyQt is not used explicitly; only as a backend for vispy.

PyQt6 does not seem to have such issues on M1, and selecting it
as a vispy extra rather than a direct dependency should be more robust.

requirements.txt

@@ -9,7 +9,6 @@ pint>=0.10
 plotly>=4.9
 pynrrd>=0.4.2
 pypng>=0.0.18
-PyQt5>=5.15
 requests>=2.20
 seaborn>=0.10
 setuptools>=50.6
@@ -18,7 +17,7 @@ six>=1.11
 tqdm>=4.45
 typing-extensions>=3.7.4
 trimesh>=3.8
-vispy>=0.6.4
+vispy[pyqt6]>=0.6.4
 fuzzywuzzy>=0.18
 molesq>=0.2.0
 rdata>=0.5