Link to pretty printer

[Alex Hall]

On the Meet the Nodes page, it says:

The pretty-printer used in these examples is available in the source repository for Green Tree Snakes.

However the link gives a 404.

This might also be a good time to mention that Python 3.9 can pretty print trees using ast.dump(…, indent=2).

[Thomas Kluyver]

Thanks, I missed that when moving from hg to git recently - default needs to become master.

[Thomas Kluyver]

Fix links to greentreesnakes' source repository

Closes #18

→ <<cset f76013bfa89d>>