python caveats: use `>` instead of `>>`

The `homebrew.pth` should be a file with a single line in it.
Also (at least on zsh) if the file does not exist, `>>` will
result in an error. The `>` works in both, bash and zsh and
creates the file with the `echo`ed contet.

Closes #28201.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>