Description: When BROWSER is set, webbrowser.get fails because register_standard_browsers throws an error.

Work around: You can still use `webbroswer._browsers[key][1].open(url)` to open a browser. 


Operating system: macOS High Sierra v10.13.6


Reproduction 1:

```
➜ BROWSER=lynx python3

Python 3.7.0 (default, Jul 23 2018, 20:22:55)
[Clang 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import webbrowser
>>> webbrowser.get()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/webbrowser.py", line 42, in get
    register_standard_browsers()
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/webbrowser.py", line 567, in register_standard_browsers
    cmd = _synthesize(cmdline, -1)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/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
```


Reproduction 2:

```
➜  BROWSER=lynx python3

Python 3.7.0 (default, Jul 23 2018, 20:22:55)
[Clang 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import webbrowser
>>> webbrowser.register_standard_browsers()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/webbrowser.py", line 567, in register_standard_browsers
    cmd = _synthesize(cmdline, -1)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/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
>>> webbrowser.lynx
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'webbrowser' has no attribute 'lynx'
>>> webbrowser._browsers
{'macosx': [None, <webbrowser.MacOSXOSAScript object at 0x10df412e8>], 'chrome': [None, <webbrowser.MacOSXOSAScript object at 0x10df41358>], 'firefox': [None, <webbrowser.MacOSXOSAScript object at 0x10df485c0>], 'safari': [None, <webbrowser.MacOSXOSAScript object at 0x10df48b70>], 'lynx': [None, <webbrowser.GenericBrowser object at 0x10e014198>]}
>>> webbrowser._browsers['lynx'].open('http://google.com')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'open'
>>> webbrowser._browsers['lynx'][1].open('http://google.com') # This works
True
```