TableEditor headings are cutoff on Windows in PyQt

The headings in the `TableEditor` do not appear to be correctly re-sizing to the displayed text. This can be seen in when running the current `TableEditor_demo.py`. Below you can see that the "g" in "Age"` is cutoff.

This gets exacerbated if you increase the font size of the application with something like:
```
qApp = QtGui.QApplication.instance()
qApp.setStyleSheet(
textwrap.dedent("""\
* { font-size: 32px }
""")
)
```

Windows 10
Python 3.6.9
TraitsUI: 6.1.3
PyQt: 5.7.1
Qt: 5.6.2