remove astropy_init

[braingram]

This removes _astropy_init.py. It is leftover from a no longer maintained project template:
https://github.com/astropy/package-template
and contains:

• code to provide asdf_astropy.__version__ (moved to __init__.py in this PR)
• code to (attempt to use) the astropy TestRunner vai asdf_astropy.test()

The TestRunner will soon be deprecated and attempting to use it on main fails with:

PytestAssertRewriteWarning: Module already imported so cannot be rewritten: asdf

Our documentation already describes ways to run the tests (none of which use asdf_astropy.test):

asdf-astropy/README.rst

Line 93 in 02e970f

Testing

Since asdf_astropy.test is not working and not documented this PR removes it.

Closes #262

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.50%. Comparing base (7adb9a5) to head (bcd5cf1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          62       62           
  Lines        2208     2209    +1     
=======================================
+ Hits         2197     2198    +1     
  Misses         11       11           

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