Pypy fixes

[mattip]

two simple fixes for PyPy:

1. When PyType_Ready is called, function slots like tp_as_number.nb_add are wrapped into a Python function and put into the Python type object. Any that are NULL are ignored. Later, when umath is imported, these are overloaded for scalars. Before this patch the functions were NULL when PyType_Ready was called, so PyPy was ignoring the overload.
2. PyPy has no Makefile

[njsmith]

I feel like a try/except might be more appropriate than an explicit pypy check? That way we're robust against pypy adding a makefile, or being used with some other python implementation that also doesn't have a makefile.

[charris]

Please squash the commits and follow the commit message template in doc/source//dev/gitwash/development_workflow.rst.

[shoyer]

Instead of checking the error message, would using an else block work? e.g.,

try:
    get_filename = sc.get_makefile_filename
except AttributeError:
    pass  # no Makefile
else:
    sc.parsefile(get_filename(), g)

It's generally a good idea to contain no more than the single line at which the exception can occur in the try block.

[mattip]

Updated as per shoyer's comment

[njsmith]

LGTM, thanks @mattip