DOC: Instead use regexp in Makefile command for API docs

N.B., the new command indicates what *not* to parse using a regular
expression rather than listing particular files and directories.

This has two consequences:

* New Python files under tulip/ will automatically be included when
  building the API documentation via Epydoc;

* If a new file is incorrectly being missed, then the regular expression
  as given in the --exclude= argument to epydoc may need to be tightened.