Astropy now depending on the Expat library to fix internal conflict

Until now, Astropy was instructed to build its own internal copy of the
Expat library. However, with the recent commits before, Maneage now
includes an installation of Expat and Astropy can't keep the two (its
internal version and the project's version) separate, so they conflict and
don't let Astropy get built.

With this commit, the problem is fixed by setting the Expat library as an
explicit dependency of Astropy and asking Astropy to ignore its internal
copy.

While doing this, I recognized that it is much easier and elegant to add
steps in various stages of the `pybuild' function through hooks instead of
variables. So the fifth argument of the `pybuild' function was removed and
now it actually checks if hooks are defined as functions and if so, they
will be called.

The `pyhook_after' function was also implemented in the installation of
`pybind11' (which needed it, given that the 5th argument of `pybuild' was
removed) and after doing a test-build, I noticed that two lines were not
ending with a `\' in `boost' (a dependency of `pybind11').

Commit written originally by Mohammad Akghlaghi

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