Move streaming dependencies to their own "extra"

[axlan]

This is a PR as a follow up to: #45

This will by default not include the numpy, Pillow, and opencv dependencies with the library install. Those will be installed with:
pip install reolinkapi[streaming]

My test was to run the library locally with a virtualenv:

Test without extra

Install:

pip install -e .

Run:

Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import reolinkapi
>>> cam = reolinkapi.Camera(HOST, USER, PASS)
Login success
>>> cam.open_video_stream()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/axlan/src/reolinkapipy/reolinkapi/mixins/stream.py", line 60, in open_video_stream
    raise ImportError('''open_video_stream requires streaming extra dependencies\nFor instance "pip install reolinkapi[streaming]"''')
ImportError: open_video_stream requires streaming extra dependencies
For instance "pip install reolinkapi[streaming]"

Test with extra

Install:

pip install -e '.[streaming]'

Run:

Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import reolinkapi
>>> cam = reolinkapi.Camera(HOST, USER, PASS)
Login success
>>> cam.open_video_stream()
opening stream

[Benehiko]

Hey @axlan this looks great! :)

[Benehiko]

You can also add some documentation on it if you'd like inside this repository :)
https://github.com/ReolinkCameraAPI/reolinkcameraapi.github.io

[Benehiko]

I have also made a new release on pypi 🎉
Thank you for the contribution, really appreciate it :)