No expire value raises error

[aspedrosa]

If no value is provided to the expire argument of the RWLock class, the acquire method raises an error.
This happens because the command EXPIRE doesn't accept the None value as argument.

On this PR I also replaced the str.format expressions by f-strings. I you don't agree with this last change I can revert it

>>> import redis
>>> redis_conn = redis.Redis()
>>> from redis_rw_lock import RWLock
>>> read_lock = RWLock(redis_conn, "some_name", RWLock.READ)
>>> read_lock.acquire()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis_rw_lock/__init__.py", line 47, in acquire
    return self.__reader_acquire()
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis_rw_lock/__init__.py", line 30, in __reader_acquire
    self.__read_switch.acquire(self.__no_writers)
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis_rw_lock/__init__.py", line 81, in acquire
    self.__redis_conn.expire(self.__counter_name, self.__expire)
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis/client.py", line 1591, in expire
    return self.execute_command('EXPIRE', name, time)
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis/client.py", line 900, in execute_command
    conn.send_command(*args)
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis/connection.py", line 725, in send_command
    self.send_packed_command(self.pack_command(*args),
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis/connection.py", line 775, in pack_command
    for arg in imap(self.encoder.encode, args):
  File "/home/aspedrosa/.trash/test/venv/lib/python3.8/site-packages/redis/connection.py", line 119, in encode
    raise DataError("Invalid input of type: '%s'. Convert to a "
redis.exceptions.DataError: Invalid input of type: 'NoneType'. Convert to a bytes, string, int or float first.