doc request: python 3.7+ specified as a requirement

[lidocaineus]

I think this doesn't init correctly in Python 3.5 (I only have one Py 3.5 environment to test on); if that's expected, can the README.md be updated?

Init error:

Python 3.5.3 (default, Apr  5 2021, 09:00:41) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import broadlink
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.5/dist-packages/broadlink/__init__.py", line 6, in <module>
    from .alarm import S1C
  File "/usr/local/lib/python3.5/dist-packages/broadlink/alarm.py", line 2, in <module>
    from .device import device
  File "/usr/local/lib/python3.5/dist-packages/broadlink/device.py", line 154
    info.append(f"{self.host[0]}:{self.host[1]}")
                                               ^
SyntaxError: invalid syntax