Deprecate Artist.get_cover_image, they're no longer available from Last.fm

[hugovk]

Fixes #317.

Changes proposed in this pull request:

• Last.fm announced that artist images were never meant to be available, in line with their API Terms of Use
• They replaced the images with default star images
• We therefore don't need to maintain Artist.get_cover_image in the future, so let's deprecate it first

[codecov]

Codecov Report

Merging #332 into master will decrease coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   94.93%   94.46%   -0.48%     
==========================================
  Files           1        1              
  Lines        1264     1265       +1     
==========================================
- Hits         1200     1195       -5     
- Misses         64       70       +6     

Impacted Files	Coverage Δ
src/pylast/__init__.py	94.46% <100.00%> (-0.48%)	⬇️

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