use requests library instead of 'curl' and obtain all comments

[trz42]

fixes #356

PR looks much bigger than it actually is. For most lines, only indentation has been reduced by one level / 4 spaces (previous version: lines 1217 - 1356, new version: lines 1229 - 1368)

Other changes are:

• imports requests library
• replaces the existing logic relying on the curl command by using the requests library plus using the Link header in the response for obtaining the next page of the paginated response (if PR has more than 100 comments) and handles an exception if any happens while querying GitHub's REST API
• variable that contains all comments changed from comments to all_comments
• old lines 1358-1359 are not needed any longer