chore(deps): bump skyfield from 1.20 to 1.21

[dependabot-preview]

Bumps skyfield from 1.20 to 1.21.

Changelog

Sourced from skyfield's changelog.

1.21 — 2020 May 29

• Added ~skyfield.positionlib.ICRF.is_sunlit() to determine whether Earth satellites in orbit are in Earth’s shadow or not, thanks to a pull request from Jesse Coffey.
• Added ~skyfield.positionlib.position_of_radec() to replace the poorly designed ~skyfield.positionlib.position_from_radec().
• Skyfield ~skyfield.timelib.Time objects now have microsecond internal accuracy, so round trips to and from Python datetimes should now preserve all the microsecond digits.
• The ~skyfield.timelib.Time.utc_strftime() method now rounds to the nearest minute or second if it sees that either minutes or seconds are the smallest unit of time in the format string.
• The 6 numbers in the sequence t.utc can now be accessed by the attribute names year, month, day, hour, minute, and second.
• Nutation routines should now be faster and have a smaller memory footprint, thanks to a rewrite that uses more optimized NumPy calls. #373
• Thanks to Jérôme Deuchnord, the exception raised when asking for a position out-of-range of a JPL ephemeris now shows the calendar dates for which the ephemeris is valid and carries several useful attributes. #356

Commits

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[codecov]

Codecov Report

Merging #98 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   86.30%   86.30%           
=======================================
  Files           8        8           
  Lines         584      584           
  Branches       95       95           
=======================================
  Hits          504      504           
  Misses         61       61           
  Partials       19       19           

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