Skyfield-data

* Upgraded `finals2002A.all` file, which has expired. Thanks a lot to ReimarBauer for the PR, and thierry-FreeBSD for the bug report.
* Dropped support for Python 3.6.
* Confirm support for Python 3.11.

* Switching from Travis/CircleCI for tests and CI to Github Actions (inspired by 24, thank you a lot, Deuchnord).
* Dropped support for Python 3.5.
* Confirmed support for Python 3.10.
* **NOTE**: Temporarily skipped Python 2.6 tests. Since version 3.0.0, the runtime code hasn't been modified, so this means that it should still be compatible, although... We can't prove it.

Data updates

**Data files were downloaded on 2022-12-15.**

* Upgrade `finals2000A.all` file, which has expired, thanks 6ruff for the bug report (20).

Important changes

* Using `finals2000A.all` file for timescale computations (14).
* Removed `Leap_Second.dat` file from the archive. They're not used by Skyfield anymore (14).
* Adapted tests for warnings to point at the `de421.bsp` file (14).
* Removed references to `Leap_Second.dat` from doc and test files (14).
* Confirmed support of Python 3.9 (15).

Data updates

**Data files were downloaded on 2020-12-18.**

* Downloaded the `finals2000A.all` data file.

Important changes

* Removed `deltat.data` & `deltat.preds` files from the archive. They're not used by Skyfield anymore, as of v1.31 (October 2020), since they were not updated by the providers. Skyfield now uses a builtin, the `iers.npz` file, embedded into the library.
* Adapted tests for warnings to point at the `Leap_Second.dat` file.
* Removed references to the `deltat.*` files in the docs and test files.

Data updates

**Data files were downloaded on 2020-12-11.**

* Updated the `Leap_Second.dat` data file.

Python compatibility

* Added Python 3.8 compatibility (1).
* Added Python 2.6 compatibility + circle-ci job, documentation amended, code fixed to be compatible with Python 2.6 (6).
* Removed tox.ini reference to Python 3.3 builds.

Minor changes

* Revamped/Simplified Travis configuration.

Data updates

**Data files were downloaded on 2020-05-05.**

* Updated ``deltat.data`` data file.
* Updated ``Leap_Second.dat`` data file.
* All expiration data items are up-to-date as of 2020-05-05.

Library Runtime Enhancements

* Added ``expiration_limit`` argument for ``get_skyfield_data_path`` function. Enables to shift the expiration date limit by "n" days.

Downloader enhancement

* USNO file serving host has changed. Pointing now at ``ftp://cddis.nasa.gov/products/iers/`` for ``deltat.*`` files.

Minor changes

* Changes in Python ``setup.cfg`` classifiers.