Replace abandoned httplib2, use charset from web server and use mocked http requests

[DerDreschner]

This pull request makes a few changes:

• httplib2 has been abandoned for ~2 years now, and it looks like it may raise problems with future Python versions. I replace it here with urllib3 to make mocked HTTP requests easy.
• The charset of a downloaded ICS file can be different from UTF-8 (like in Add encoding parameter to events, etc. #104). Therefore, we should use the charset transmitted by the web server instead of guessing it's UTF-8.
• The unit tests used real HTTP requests against the master branch here on GitHub so far. This makes testing different cases quite impossible. As I want to see if downloading files with different charsets works, I had to change this. Therefore, I implemented mocked HTTP requests using pock.
• Implement test cases for Latin-1 ICS files as well as handling empty responses correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.86%. Comparing base (97493e3) to head (0570410).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   91.31%   91.86%   +0.54%     
==========================================
  Files           4        4              
  Lines         380      381       +1     
  Branches       94       94              
==========================================
+ Hits          347      350       +3     
+ Misses         15       14       -1     
+ Partials       18       17       -1     

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