Catch the ValueError in both invocations of DepsReader._read.

Added a docstring to _read to explicitly declare the intended interface. Closes #105

pip_run/scripts.py

@@ -154,7 +154,7 @@ def read_python(self):
         raw_reqs = suppress(ValueError)(self._read)('__requires__') or []
         reqs_items = jaraco.text.yield_lines(raw_reqs)
         deps = Dependencies.load(reqs_items)
-        with contextlib.suppress(Exception):
+        with contextlib.suppress(ValueError):
             deps.index_url = self._read('__index_url__')
         return deps