MNT: Replace master with main

Make Skyfield correct for polar motion

FTP [doesn't work in
Travis](https://docs.travis-ci.com/user/common-build-problems/#ftpsmtpother...

This replaces the deprecated Topos object, which apparently used the
IERS2010 ellipsoid rather t...

By default Skyfield doesn't correct for polar motion; it needs to be
told to fetch the IERS tabl...

Run benchmarks in Python 3.7

Fix gh-67.

Remove ancient outputs

Attempt to fully fix gh-69.

Fix publish

Fix gh-69.

Use GitHub Actions to run benchmarks

Publish results from Python 3 and drop Python 2

See gh-66.

Fix gh-64.

Partial support for Skyfield

- Use an explicit https URL to get DE430, instead of letting Skyfield
default to FTP.
- Use wg...

It implements only convert_horizontal. I haven't tested whether this
causes problems due to the ...

This is consistent with what Astropy uses.

Remove matplotlib calls that had no effect

Unify Python 2 and Python 3 requirements

Works with pip as old as 9.x

Auto-deploy from Travis

Enable ecliptic for astropy

Kapteyn version update to 2.3

v2.2 is no longer at that URL.

Fix URL of output plots

The output path for the plots is not the same as the URL for the
plots because the HTML is loade...

Use ci helpers for Travis

Add support for horizon coords using pyast

Include PySOFA?

Restructure coordinate-benchmarks