Add support for write only properties

[TD22057]

Issue description

We have a few cases where we have C++ code that has write only properties (just a set method) that we'd like to promote. These are basically identical to def_property_readonly except with just a set method. The current def_property can't be used because it relies on the read function always being there to get docs. The additions we made are:

• def_property_writeonly
• def_property_writeonly_static

If you don't think this is appropriate, feel free to close this. Otherwise, I'll submit a PR w/ the updated functions and test cases that we've implemented. If you think it's necessary, I'm sure we could also add def_writeonly and def_writeonly_static for completeness.

[jagerman]

It seems fine to me; you can do something like this in pure Python via:

Python 3.5.4 (default, Sep  5 2017, 18:32:10) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class C():
...     def set_var(self, v):
...         self._var = v
...     var = property(None, set_var)
... 
>>> c = C()
>>> c.var
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: unreadable attribute
>>> c.var = 123
>>> 

so it'd be nice in a PR to duplicate that unreadable attribute error, but aside from that it seems a reasonable feature.

[jagerman]

Fixed via #1144.