Usage of Python 2 `__metaclass__`

[auscompgeek]

There are 7 instances of __metaclass__ in Synapse, which does nothing in Python 3.
These should probably be changed to use class Foo(metaclass=abc.ABCMeta).

synapse/handlers/room_member.py
48:    __metaclass__ = abc.ABCMeta

synapse/server.py
141:    __metaclass__ = abc.ABCMeta

synapse/replication/http/_base.py
77:    __metaclass__ = abc.ABCMeta

synapse/storage/data_stores/main/account_data.py
41:    __metaclass__ = abc.ABCMeta

synapse/storage/data_stores/main/receipts.py
41:    __metaclass__ = abc.ABCMeta

synapse/storage/data_stores/main/push_rule.py
80:    __metaclass__ = abc.ABCMeta

synapse/storage/data_stores/main/stream.py
251:    __metaclass__ = abc.ABCMeta

[auscompgeek]

Probably also worth noting that flake8-bugbear lints for this.