Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

This tries to clarify the type alias deprecation message slightly
to give more guidance on how to review the replacement (if desired).

Most end-users will probably not care about the warning in any case,
since they will not just import the function but also use it.
We can't just remove it, due to gh-17143: Astropy currently pulls
in these functions (but doesn't use them), so that plain removal
would lead to an unusable astropy if a new NumPy is installed.
Even more annoying, due to a (faulty?) astropy pytest plugin, this
affects all pytest runs (which do not use `PYTEST_DISABLE_PLUGIN_AUTOLOAD`).

Changing it to a DeprecationWarning seems to remove the issue from
pytest runs, this may make the warning less visible in rare cases
where it should be seen, but hopefully it will still be visible enough.

In a previous commit, the expired financial functions were removed
from NumPy, and code was added to __init__.py using the module
__getattr__ to raise a customized AttributeError on any attempt to
reference the expired names in the numpy namespace.

That change broke released versions astropy, which has code that
imports the financial functions.  astropy never calls the functions,
so they never saw the deprecation warnings that have been in place
since numpy 1.18.  This means that attempting to use a released
version of astropy with numpy 1.20 will cause astropy to crash
with the custom AttributeError.

In this commit, instead of raising an exception when one of the
expired names is referenced, a warning is generated.  If the
function is *called*, an exception is raised.

As explained in NEP 32, the financial functions are to be removed
from version 1.20.
They are now replaced with module level `__getattr__` to give a useful
error message for those surprised by the `AttributeError`.

This only works for Python 3.7+, but it is expected that by the 1.20 release
Python 3.6 will not be supported.