Removed --enable-legacy option
Add PyPI shields to README
changes xorarr back to x for cleaner API
Trial with equivalency=(u.rad,None)
Add radian_is_dimensionless function to equivalencies.py
Add test cases, and, as a result, correct is_equivalent
Also rename equivalency to angles_dimensionless
Add documentation for angles_dimensionless in units/equivalencies.rst
Correct equivalency tests
Use same test for (unit, None) equivalency in is_equivalent and _apply_equivalencies
More reliable test for (unit, None) converter
Change build-in equivalency test to one that uses a class
This since objects can no longer be initialised with equivalencies
Change name to dimensionless_angles
Remove xfail equivalency context manager test given renaming in #1662
Largely trivial whitespace and spelling cleanup, with renames to a few local variables and a few comments with TODO items.
Move the indent() utility function into astropy.utils and use it to make some of these __repr__ methods look a little nicer.
Some changes to function/variable/argument/attribute names for better
naming consistency:

1. xdeg -> x_degree, etc. for consistency with other names like x_window
   each variable like this places an underscore after the dimension
   label and spells out the property associated with that dimension

2. pars -> params: not spelling out "parameter" constantly seems fine
   but a consistent abbreviation should be chosen. I want with "params"
   over "pars" which seems a little more explicit

3. In the `Parameter` class: parshape -> shape (the "par" part seems
   superfluous). Likewise param_dim -> dim; I realize param_dim was
   consistent with the param_dim attribute on Models, but it still
   seems superfluous to read things like `param.param_dim`.
   Finally mclass -> model. The name "mclass" suggests a class, not an
   instance of a model.  I thought of something like "modelobj" or
   "modelinst" but finally "model" just seemed clear enough.

4. cons -> constraints; "cons" as an abbreviation is a little ambiguous
   IMO

5. OrthogPolyBase -> OrthoPolynomialBase; preferred to spell out
   Polynomial same as in PolynomialModel. Also 'Ortho' just rolls
   off the tongue a little easier to me than 'Orthog' but this is
   admittedly highly subjective....
Fix some double list copies
Moved the ModelDefinitionError and InputParameterError out of utils and closer to their relevant modules. Also removed unecessary extra structure to their class definitions.
Defined an EulerAngleRotation base class for RotateNative2Celestial and its inverse, containing a lot of shared functionality.
Convert these methods to staticmethods since they don't need to use the class or instances of the class they're defined in directly (they're just the definitions of the functions these models are wrapping).  The one exception was Box1D where it was useful to classmethod.