Arch Linux is also affected.
Steps to reproduce:
```
$ python
Python 3.7.0 (default, Jul 15 2018, 10:44:58) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import webbrowser
>>> webbrowser.get(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/webbrowser.py", line 42, in get
    register_standard_browsers()
  File "/usr/lib/python3.7/webbrowser.py", line 567, in register_standard_browsers
    cmd = _synthesize(cmdline, -1)
  File "/usr/lib/python3.7/webbrowser.py", line 116, in _synthesize
    register(browser, None, controller, update_tryorder)
TypeError: register() takes from 2 to 3 positional arguments but 4 were given
>>> 
```

Workaround:
```
$ env BROWSER='' python
Python 3.7.0 (default, Jul 15 2018, 10:44:58) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import webbrowser
>>> webbrowser.get(None)
<webbrowser.Mozilla object at 0x7f6bf8c75400>
>>> 
```