Dyn methods: Factor out instance var

This small refactoring prepares for a later commit that will switch to
binding the methods to the class instead of to the instance. The
instance attribute `prog` will not be available then anymore, because
no instance exists yet at the time the class is created. Therefore, the
reference is now replaced by the string `'dot'`.

Note that this does not actually change the default value at all. It
always got fixed to the value of `prog` at the time of function
definition, which was always `'dot'`, as can be seen from line 1713.
The default value already never changed along with the instance
attribute value anymore after function definition.

Side note:

- One could argue that `None` would make a better default value here.
  However, changing that now would be changing the external behavior of
  the methods themselves, while this series aims to change the way in
  which they are created with minimal external effects. I plan to
  propose a switch to `None` together with other changes to
  `create_*`/`write_*` in another series, though.

(This commit is part of a series of changes in how the `get_*`,
`set_*`, `create_*` and `write_*` methods are dynamically created.)

pydot.py

@@ -1717,7 +1717,7 @@ def __init__(self, *argsl, **argsd):
         # of output in any of the supported formats.
         for frmt in self.formats:
             def new_method(
-                    f=frmt, prog=self.prog,
+                    f=frmt, prog='dot',
                     encoding=None):
                 """Refer to docstring of method `create`."""
                 return self.create(
@@ -1727,7 +1727,7 @@ def new_method(
 
         for frmt in self.formats+['raw']:
             def new_method(
-                    path, f=frmt, prog=self.prog,
+                    path, f=frmt, prog='dot',
                     encoding=None):
                 """Refer to docstring of method `write.`"""
                 self.write(