In most pygtk widget pages, they contain sections called 'Attributes', 'Properties', and 'Style Properties'. How can I change these properties and attributes?
There are three ways to change properties:

As in zheoffec's answer, use the set_property() function (or set_style_property() for style properties.) This function is actually not necessary in Python, but it is there for completeness because it is part of the C API.
Use the props attribute. Any property that you find in the documentation can be accessed through this attribute. For example, btn1.props.label = 'StackOverflow' and btn1.props.use_underline = False.
Use the getter and setter functions as frb suggests. These are also only present because they are part of the C API, but some people prefer them over the props attribute. Also, there is no guarantee that any particular property will have getter and setter functions! Usually in a well-designed C API they will be there, but it is not required.

For style properties, I believe the only option is #1. For "attributes", well, these are simply Python attributes. To access the allocation attribute, use btn1.allocation.
Editing GtkWidget attributes/properties