How to change a repo source?

[Skip Montanaro]

The only use I make of Mercurial is to maintain a series of clones of
the different Python release branches and the current tip. I then have
a shell script I execute to update everything in order, executing hg
pull, configure, make altinstall.

Those repositories are organized in a bucket brigade:

* cpython pulls from the default on hg.python.org
* 3.4 pulls from cpython
* 3.3 pulls from 3.4
* etc

I just executed

hg clone cpython 3.5

I suppose I can keep 3.4 pointed at cpython, but it seems the only
time it will get an update is if a bug fix is applied to 3.5. How do I
point my 3.4 repo at 3.5 instead of cpython?

Thanks,

Skip Montanaro