Change default method in Location.get_sun_rise_set_transit

[AdamRJensen]

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I would like to suggest that we switch the default method of pvlib.location.Location.get_sun_rise_set_transit to spa instead of pyephem.

The main reason is that I think it's annoying that we're defaulting to a method that requires the installation of an optional external package when we have functions within pvlib that can do this.

Also, as pointed out by @cwhanse in #1631 (comment), the pyephem package is in maintenance-only mode, which is another reason not to rely to much on this.