A collection of doctest examples that should be run in a single
namespace. Each `DocTest` defines the following attributes:
- examples: the list of examples.
- globs: The namespace (aka globals) that the examples should
be run in.
- name: A name identifying the DocTest (typically, the name of
the object whose docstring this DocTest was extracted from).
- filename: The name of the file that this DocTest was extracted
from, or `None` if the filename is unknown.
- lineno: The line number within filename where this DocTest
begins, or `None` if the line number is unavailable. This
line number is zero-based, with respect to the beginning of
the file.
- docstring: The string that the examples were extracted from,
or `None` if the string is unavailable.