Fix some things that will break in 3.9, but are still compatible with 3.7.

Examples:

• importing ABC from collections is removed, use from abc

• macpath module removed

• platform.popen → os.popen

• time.clock removed, use time.perf_counter

• pyvenv removed, use python -m venv

• filemode removed from tarfile module

• XMLParser no longer accepts html argument

• Removed doctype from XMLParser

• array: tostring() and fromstring() removed

• base64 encodestring and decodestring removed (now encodebytes/decodebytes)

• json.loads() no longer accepts encoding param

• unescape removed from HTMLParser, use html.unescape

• _dummy_thread and dummy_threading modules removed

• __import__() now raises ImportError instead of ValueError

• Unexpected errors in calling the __iter__ method are no longer masked by TypeError