Raise exception for astropy <4 in __init__.py

[dlakaplan]

Fixes #1354

astropy >=4 is enforced by requirements.txt

[paulray]

I guess this is OK, but we've definitely had people install PINT, then upgrade via git pull and never need to run pip again so that requirements.txt doesn't get applied. This happened to many of us with loguru, for example.

[dlakaplan]

If that's the case, then probably raising an exception is better than emitting a warning.

[paulray]

Indeed, especially for something as old as astropy 3.

[dlakaplan]

Now astropy < 4 raises an exception, and there is a test for that.

[codecov]

Codecov Report

Merging #1359 (cd72d68) into master (a8daf80) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1359      +/-   ##
==========================================
+ Coverage   61.29%   61.30%   +0.01%     
==========================================
  Files          90       90              
  Lines       20349    20351       +2     
  Branches     3646     3646              
==========================================
+ Hits        12473    12477       +4     
+ Misses       7089     7088       -1     
+ Partials      787      786       -1     

Impacted Files	Coverage Δ
src/pint/__init__.py	97.36% <100.00%> (+5.26%)	⬆️
src/pint/models/frequency_dependent.py	83.33% <0.00%> (+0.47%)	⬆️

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