Fix: remove numpy.str usage in MDSplus

[santorofer]

In MDSplus python/mdsscalar.py, in the String class, resides the only usage of the numpy.str method. This method has been deprecated in numpy versions >= 1.24.

The fix consists in using Python's own str method instead.

[santorofer]

Tests:

BEFORE the fix:

fsantoro at mfews-fsantoro in ~ 
$ python3
Python 3.8.10 (default, Nov 14 2022, 12:59:47) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import MDSplus
>>> MDSplus.String('a string')
/usr/local/mdsplus/python/MDSplus/mdsscalar.py:422: FutureWarning: In the future `np.str` will be defined as the corresponding NumPy scalar.  (This may have returned Python scalars in past versions.
  if not isinstance(self._value, _N.str):
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/mdsplus/python/MDSplus/mdsscalar.py", line 422, in __init__
    if not isinstance(self._value, _N.str):
  File "/home/fsantoro/.local/lib/python3.8/site-packages/numpy/__init__.py", line 284, in __getattr__
    raise AttributeError("module {!r} has no attribute "
AttributeError: module 'numpy' has no attribute 'str'
>>> 

AFTER the fix:

fsantoro at mfews-fsantoro in ~ 
$ python3
Python 3.8.10 (default, Nov 14 2022, 12:59:47) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import MDSplus
>>> MDSplus.String('a string')
'a string'
>>>