Run doctests without implicit imports under pytest/doctestplus

[oscarbenjamin]

@drdavella @asmeurer

Recently merged #15768 makes it possible to run doctests under pytest using the pytest-doctestplus plugin from astropy. Sympy's existing (internal) doctest runner runs doctests differently from the stdlib doctest module, the pytest default doctest plugin, and the doctestplus plugin.

Specifically the difference is that Sympy's doctest runner requires all names to be explicitly imported. The other doctest runners (following the stdlib) all provide implicit imports from the defining module so e.g. you can do

# mod.py

def func():
    '''
    >>> func()
    1
    '''
    return 1

and the test will pass. Sympy's test runner would treat the other as an error and require instead:

# mod.py

def func():
    '''
    >>> from mod import func
    >>> func()
    1
    '''
    return 1

It is not currently possible to have this behaviour with doctestplus but I don't think it would be hard to add support for it. I think it just amounts to setting globs to an empty dict here:
https://github.com/astropy/pytest-doctestplus/blob/master/pytest_doctestplus/plugin.py#L376
There would need to be some setting in doctestplus to control this though.