You can use v4l2loopback (https://github.com/umlaeute/v4l2loopback) for that. It is written in C but there are some python wrappers. I've used virtualvideo (https://github.com/Flashs/virtualvideo).
The virtualvideo README is pretty straight forward but here is my modification of their github example to match your goal:

import virtualvideo
import cv2


class MyVideoSource(virtualvideo.VideoSource):
    def __init__(self):
        self.cam = cv2.VideoCapture(0)
        _, img = self.cam.read()
        size = img.shape
        #opencv's shape is y,x,channels
        self._size = (size[1],size[0])

    def img_size(self):
        return self._size

    def fps(self):
        return 30

    def generator(self):
        while True:
            _, img = self.cam.read()
            yield img


vidsrc = MyVideoSource()
fvd = virtualvideo.FakeVideoDevice()
fvd.init_input(vidsrc)
fvd.init_output(2, 640, 480, pix_fmt='yuyv422')
fvd.run()

in init_output the first argument is the virtual camera resource thar i've created when adding the kernel module:

sudo modprobe v4l2loopback video_nr=2 exclusive_caps=1

the last arguments are my webcam size and the pixel format.

You should see this option in hangouts:

check out this project: https://github.com/letmaik/pyvirtualcam

This may be late but check this out pyvirtualcam

install it first:

pip install pyvirtualcam

You can make it like this:

import pyvirtualcam
import numpy as np

with pyvirtualcam.Camera(width=1280, height=720, fps=30) as cam:
    while True:
        frame = np.zeros((cam.height, cam.width, 4), np.uint8) # RGBA
        frame[:,:,:3] = cam.frames_sent % 255 # grayscale animation
        frame[:,:,3] = 255
        cam.send(frame)
        cam.sleep_until_next_frame()