ponysay: fix SyntaxWarning

Currently when running ponysay, you get this warning twice:

```
/nix/store/.../bin/ponysay/balloon.py:43: SyntaxWarning: invalid escape sequence '\-'
```

It is caused by a docstring containing twice the phrase `\-directional`.

The fix is just to make it a raw string (replace `'''` with `r'''`).

pkgs/by-name/po/ponysay/package.nix

@@ -23,6 +23,9 @@ stdenv.mkDerivation rec {
     substituteInPlace setup.py --replace \
         "fileout.write(('#!/usr/bin/env %s\n' % env).encode('utf-8'))" \
         "fileout.write(('#!%s/bin/%s\n' % (os.environ['python3'], env)).encode('utf-8'))"
+    substituteInPlace src/balloon.py --replace-fail \
+        $'\'\'\'\n        Constructor' \
+        $'r\'\'\'\n        Constructor'
     python3 setup.py --prefix=$out --freedom=partial install \
         --with-shared-cache=$out/share/ponysay \
         --with-bash