Notable changes

• Add Python 3.8 support, drop 3.3 and 3.4. Python 3.5 is now the minimum required version.
• Add Pixelfed! Heavily based on Mastodon.
• Standardize Instagram's and Facebook's scraping into new common scraped_to_activities(), scraped_to_activity(), and merge_scraped_reactions() methods.
• Atom:
  • Add the summary element (#157).
• REST API:
  • Bug fix: URL-encode Unicode characters in Link HTTP headers (eg rel=self, rel=header).
• Facebook:
  • Scraping now uses mbasic.facebook.com instead of m.facebook.com.
• Flickr:
  • Add support for adding tags to existing photos (bridgy#857).
  • get_comment(): skip fetching comments from API if activity kwarg is provided and contains the requested comment.
• GitHub:
  • Handle HTTP 451 Unavailable for Legal Reasons responses (eg for DMCA takedowns) gracefully.
  • Add create/preview support for reactions on pull review request comments (ie URLs with #discussion_r... fragments).
• HTML/microformats2:
  • Add aria-hidden="true" to empty links (bridgy#947).
  • Bug fix: escape &, <, and > characters in bare mf2 content properties (aaronpk/XRay#102).
  • json_to_object(): convert nickname to username.
• JSON Feed:
  • Gracefully handle when content_html and content_text are incorrectly lists instead of strings.
• Instagram:
  • Include threaded (ie nested) comments in scraping (bridgy#958).
• Mastodon:
  • Bug fix for alt text with image attachments (bridgy#975).
  • Omit empty limit param for compatibility with Pleroma (bridgy#977).
• Meetup:
  • create(): handle API errors and return the error message in the CreationResult (bridgy#921).
• Twitter:
  • Bug fix: URL-encode list names in API calls.
  • Bug fix: propagate alt text into AS1 photo.displayName so that it gets all the way into microformats2 JSON and HTML (#183).
• Reddit:
  • Implement post_id().
  • Cache user data fetched from the API for 5m to avoid repeating user profile API requests (bridgy#1021). when fetching multiple comments or posts from the same author
  • Bug fix: use 'displayName' instead of 'name' in AS1 objects for submissions.
  • Bug fix: use tag URIs for activity ids.
• ActivityStreams 2:
  • to_as1(): for Create activities, include the activity actor's data in the object's author (snarfed/bridgy-fed#75).
  • to_as1(): convert preferredUsername to username.
  • from_as1(): convert username to preferredUsername.
  • from_as1(): bug fix, make context kwarg actually work.