API: ensure no methods have any docstrings when `PYTHONOPTIMIZE=2`
API: ensure no methods have any docstrings when `PYTHONOPTIMIZE=2`

Previously the docstrings were assigned to after creating functions
for the methods, thus `PYTHONOPTIMIZE` was ignored. The previous
behavior did not comply to what `PYTHONOPTIMIZE=2` means [1].

To observe the previous behavior:

```
$ python -OO
>>> import pydot
>>> d = pydot.Dot()
>>> assert d.create_canon.__doc__ is None
```

[1] https://docs.python.org/3/using/cmdline.html#envvar-PYTHONOPTIMIZE

pydot/pydot,erocarrera/pydot