Only test write operations on a single Python version to avoid collision failures

[hugovk]

Changes proposed in this pull request:

• Because test are run using a real (test) account on Last.fm, sometimes one of the parallel jobs changes the state on the network and causes another job to fail (eg. last master https://travis-ci.org/github/pylast/pylast/builds/701542153)
• Let's only do write tests with a single Python version/CI job (currently 3.8), to isolate potential conflicts

[codecov]

Codecov Report

Merging #333 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #333   +/-   ##
=======================================
  Coverage   94.46%   94.46%           
=======================================
  Files           1        1           
  Lines        1265     1265           
=======================================
  Hits         1195     1195           
  Misses         70       70           

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