Fix parsing of "20161101" as YYYYMMDD instead of year 20161101

[dewey]

I've noticed that some entries in my libraries had year > 9999 set and tracked it down to this being parsed wrongly by this function. I think this simple change will prevent this going forward. I added some tests to verify that and it looks good.

What do you think?

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.28%. Comparing base (7b53be9) to head (540df67).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   93.26%   93.28%   +0.01%     
==========================================
  Files          16       16              
  Lines         817      819       +2     
  Branches      118      119       +1     
==========================================
+ Hits          762      764       +2     
  Misses         35       35              
  Partials       20       20              

Files with missing lines	Coverage Δ
mediafile/fields.py	93.25% <100.00%> (+0.08%)	⬆️