Previously, the contents of a Python 'str' would be passed
directly to C++ methods that took non-const 'char *' arguments.
This was bad, since 'str' is immutable but there was no guarantee
that the C++ method wouldn't try to modify the string.  Now the
C++ method receives a copy of the Python string for 'char *' args
and only directly receives the contents for 'const char *' args.
If a bytearray or other mutable sequence is passed as 'char *',
then the wrappers will write any modifications back to Python.

In Python 3.7 the PyUnicode_AsUTF8() method was changed so that
it returns "const char *" instead of returning "char *", so this
commit is needed in order to compile VTK with Python 3.7.