requests 2.14.x erroring when using a custom HTTPSConnectionPool

This worked fine with 2.13.0 but started failing today since 2.14.0, this is the stacktrace
```
Traceback (most recent call last):
  File "requests_test.py", line 31, in <module>
    s.request(method='GET', url='https://google.com')
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/sessions.py", line 518, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/sessions.py", line 639, in send
    r = adapter.send(request, **kwargs)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/adapters.py", line 403, in send
    conn = self.get_connection(request.url, proxies)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/adapters.py", line 307, in get_connection
    conn = self.poolmanager.connection_from_url(url)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/packages/urllib3/poolmanager.py", line 279, in connection_from_url
    pool_kwargs=pool_kwargs)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/packages/urllib3/poolmanager.py", line 227, in connection_from_host
    return self.connection_from_context(request_context)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/packages/urllib3/poolmanager.py", line 240, in connection_from_context
    return self.connection_from_pool_key(pool_key, request_context=request_context)
  File "/Users/ludo/Library/Python/2.7/lib/python/site-packages/requests/packages/urllib3/poolmanager.py", line 261, in connection_from_pool_key
    pool = self._new_pool(scheme, host, port, request_context=request_context)
TypeError: _new_pool() got an unexpected keyword argument 'request_context'
```


Here's the sample code to reproduce the issue:
```
import requests
from requests.adapters import HTTPAdapter
from requests.packages.urllib3.poolmanager import PoolManager, HTTPSConnectionPool

class MyHTTPSConnectionPool(HTTPSConnectionPool):
    def __init__(self, host, port=None, **conn_kw):
        HTTPSConnectionPool.__init__(self, host, port, **conn_kw)

class MyPoolManager(PoolManager):
    def __init__(self, **kw_args):
        PoolManager.__init__(self, **kw_args)

    def _new_pool(self, scheme, host, port):
        if scheme == 'https':
            kwargs = self.connection_pool_kw
            return MyHTTPSConnectionPool(host, port, **kwargs)
        return super(MyPoolManager, self)._new_pool(scheme, host, port)

class MyAdapter(HTTPAdapter):

    def __init__(self, **kw_args):
        HTTPAdapter.__init__(self, **kw_args)
    def init_poolmanager(self, connections, maxsize, block=False):
        self.poolmanager = MyPoolManager(num_pools=connections,

s = requests.Session()
s.mount('https://', MyAdapter())
s.request(method='GET', url='https://google.com')
```

I believe this is fixed with #4007 please upgrade to 2.14.1.

I'm not sure if this is actually related to #4007. Some work was done in urllib3 1.21 to allow the use of pool_kwargs in the `connection_from_*` functions. Part of [this commit](https://github.com/shazow/urllib3/commit/7c14966fc134146223650a8393e17e91a358ba6d) introduced the passing of a new argument, `request_context` in `_new_pool`.

You'll find the function signature you're providing in your custom PoolManager no longer matches what urllib3 expects. Modifying your example code to `def _new_pool(self, scheme, host, port, request_context=None)` will fix this issue.