Errors and warnings when running the testsuite on Python 3

Apart from the test failures already reported in https://github.com/enthought/mayavi/issues/1004, a number of errors of the following type were observed -

```
======================================================================
ERROR: test_save_and_restore (mayavi.tests.test_image_data_probe.TestImageDataProbe)
Test if saving a visualization and restoring it works.
----------------------------------------------------------------------
Traceback (most recent call last):
File "c:\users\rporuri\work\github\ets\mayavi\mayavi\tests\test_image_data_probe.py", line 99, in test_save_and_restore
engine.load_visualization(f)
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\scripting\recordable.py", line 48, in _wrapper
result = func(*args, **kw)
File "c:\users\rporuri\work\github\ets\mayavi\mayavi\core\engine.py", line 295, in load_visualization
scene.__set_pure_state__(scene_state)
File "c:\users\rporuri\work\github\ets\mayavi\mayavi\core\scene.py", line 89, in __set_pure_state__
set_state(self, state, last=['scene'])
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 1001, in set_state
StateSetter().set(obj, state, ignore, first, last)
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 792, in set
self._do(obj, key, state[key])
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 875, in _do
self._do_object(getattr(obj, key), value)
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 880, in _do_object
self.type_map[state.__class__](obj, state)
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 917, in _do_list
self._do_object(obj[i], state[i])
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 880, in _do_object
self.type_map[state.__class__](obj, state)
File "C:\Users\rporuri\work\github\cp38-venv-test-ets-pkgs\lib\site-packages\apptools\persistence\state_pickler.py", line 891, in _do_instance
obj.__set_pure_state__(state)
File "c:\users\rporuri\work\github\ets\mayavi\mayavi\sources\vtk_xml_file_reader.py", line 202, in __set_pure_state__
super(VTKXMLFileReader, self).__set_pure_state__(state)
File "c:\users\rporuri\work\github\ets\mayavi\mayavi\core\file_data_source.py", line 199, in __set_pure_state__
raise IOError(msg)
OSError: Could not find file at C:\c:\users\rporuri\work\github\ets\mayavi\mayavi\tests\data\pyramid_ug.vtu
Please move the file there and try again.
```

A large number of the following vtk warning were displayed -

```
2021-04-15 12:13:28.186 ( 29.296s) [ ] vtkAlgorithm.cxx:1804 WARN| vtkAlgorithm::SetProgress was deprecated for VTK 9.0 and will be removed in a future version. Use vtkAlgorithm::UpdateProgress instead.
```

A number of resource warnings of the following kind were displayed -

```
c:\users\rporuri\work\github\ets\mayavi\mayavi\tools\data_wizards\csv_sniff.py:174: ResourceWarning: unclosed file <_io.TextIOWrapper name='c:\\users\\rporuri\\work\\github\\ets\\mayavi\\mayavi\\tests\\csv_files\\webaccess.csv' mode='r' encoding='cp1252'>
return loadtxt(self._filename, **self.kwds())
ResourceWarning: Enable tracemalloc to get the object allocation traceback
```

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
-e git+git@github.com:enthought/mayavi.git@61ef7c0eebbdca474b7cfdf6082bfc17fb5aaf59#egg=mayavi
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
vtk==9.0.1
wcwidth==0.2.5
zipp==3.4.1
```