Zeroconf

- This time really allowed incoming service names to contain
underscores (patch released as part of 0.21.0 was defective)

- Fixed import-time typing-related TypeError when older typing version
is used

- Fixed installation on Python 3.4 (we use typing now but there was no
explicit dependency on it)

- Added an error message when importing the package using unsupported
Python version
- Fixed TTL handling for published service
- Implemented unicast support
- Fixed WSL (Windows Subsystem for Linux) compatibility
- Fixed occasional UnboundLocalError issue
- Fixed UTF-8 multibyte name compression
- Switched from netifaces to ifaddr (pure Python)
- Allowed incoming service names to contain underscores

- Dropped support for Python 2 (this includes PyPy) and 3.3
- Fixed some class' equality operators
- ServiceBrowser entries are being refreshed when 'stale' now
- Cache returns new records first now instead of last

- Allowed installation with netifaces \>= 0.10.6 (a bug that was
concerning us got fixed)