Add a new doctest_namespace fixture

[milliams]

This resolves #1047 by introducing a new fixture doctest_namespace. This fixture can be used to inject names into the namespace in which your doctests run. The basic usage is described in doctest.rst but it works like:

# content of conftest.py
import numpy
@pytest.fixture(autouse=True)
def add_np(doctest_namespace):
    doctest_namespace['np'] = numpy

which can then be used in your doctests directly::

# content of numpy.py
def arange():
    """
    >>> a = np.arange(10)
    >>> len(a)
    10
    """
    pass

This is my first contribution to pytest so I welcome any feedback on style etc.

[The-Compiler]

The indentation here seems to be off 😉

[milliams]

Indeed 😄 I will fix and update.

[nicoddemus]

Thanks a lot for the PR @milliams! Excellent work! 😄