Deprecation warnings when running the testsuite on Python 3.8

The following deprecations are seen with PySide2

```
C:\Users\rporuri\work\github\ets\traitsui\traitsui\handler.py:279:
DeprecationWarning: In the future, None will no longer be accepted by this trait type. To allow None and silence this warning, use Date(allow_none=True)
```

```
C:\Users\rporuri\work\github\ets\traitsui\traitsui\qt4\helper.py:71:
DeprecationWarning: QPixmapCache.find(const QString & key) is deprecated
pm = QtGui.QPixmapCache.find(filename)
```

output of pip freeze

```
apptools==5.1.0
attrs==20.3.0
backcall==0.2.0
celiagg==2.1.1
-e git+git@github.com:enthought/chaco.git@1ed123cc65d4fd2b9a53a238cd584f8c5dfcac68#egg=chaco
colorama==0.4.4
configobj==5.0.6
Cython==0.29.22
decorator==5.0.7
enable @ file:///C:/Users/rporuri/Downloads/enable-5.1.0-cp38-cp38-win_amd64.whl
-e git+git@github.com:enthought/envisage.git@d62af08e75c5555e69c4fc4688393dee737680e8#egg=envisage
fonttools==4.22.0
hypothesis==6.9.2
importlib-metadata==3.10.1
importlib-resources==5.1.2
ipykernel==5.5.3
ipython==7.22.0
ipython-genutils==0.2.0
jedi==0.18.0
jupyter-client==6.1.12
jupyter-core==4.7.1
kiwisolver==1.3.1
mock==4.0.3
numexpr==2.7.3
numpy==1.20.2
packaging==20.9
pandas==1.2.4
parso==0.8.2
pickleshare==0.7.5
Pillow==8.2.0
prompt-toolkit==3.0.18
-e git+git@github.com:enthought/pyface.git@004d6ee43acaa51b17e78707f8aeb1118d818725#egg=pyface
pygarrayimage==1.0
pyglet==1.5.16
Pygments==2.8.1
pyparsing==2.4.7
PyPDF2==1.26.0
PyQt5-Qt5==5.15.2
PyQt5-sip==12.8.1
PySide2==5.15.2
python-dateutil==2.8.1
pytz==2021.1
pywin32==300
pyzmq==22.0.3
reportlab==3.5.67
shiboken2==5.15.2
six==1.15.0
sortedcontainers==2.3.0
tables==3.6.1
tornado==6.1
traitlets==5.0.5
-e git+git@github.com:enthought/traits.git@17c60780393d6c885d45f727209c65d8a1d458fc#egg=traits
-e git+git@github.com:enthought/traits-futures.git@9ec7f4567b06c1afd6f582301d69e87dfdbac3ee#egg=traits_futures
-e git+git@github.com:enthought/traitsui.git@8c21ab1fe9f526cf4ffcc1901739ca4661a7a450#egg=traitsui
wcwidth==0.2.5
zipp==3.4.1
```