Make RequestsClient thread-safe

It came up in #526 that our implementation isn't quite thread-safe.
*Most* HTTP clients are by virtue of the fact that their underlying
libraries are, but the recommended client, `RequestsClient` for the
Requests library, creates and reuses only a single session for all the
requests it makes.

The Requests maintainers are non-committal when it comes to the thread
safety of a session, but the general sentiment of its userbase is that
sessions are not thread safe.

Here we tweak the `RequestsClient` implementation so that we create one
session per thread by leveraging Python's thread-local storage.

Unfortunately, this implementation still leaves a pitfall in that if you
explicitly pass in a session in, it will get used between all threads.
The ideal thing to do here I think would be to probably not accept a
session object anymore, but that'd be a breaking change. One possibility
is that we could start warning about the use of the session parameter,
but for the time being I've opted to just leave this as a known problem
(given that hopefully this patch will still be a strict improvement).

stripe/http_client.py

@@ -6,6 +6,7 @@
 import email
 import time
 import random
+import threading
 
 import stripe
 from stripe import error, util, six
@@ -101,6 +102,8 @@ def __init__(self, verify_ssl_certs=True, proxy=None):
  )
  self._proxy = proxy.copy() if proxy else None
 
+ self._thread_local = threading.local()
+
  def request_with_retries(self, method, url, headers, post_data=None):
  num_retries = 0
 
@@ -191,7 +194,7 @@ class RequestsClient(HTTPClient):
  def __init__(self, timeout=80, session=None, **kwargs):
  super(RequestsClient, self).__init__(**kwargs)
  self._timeout = timeout
- self._session = session or requests.Session()
+ self._thread_local.session = session or requests.Session()
 
  def request(self, method, url, headers, post_data=None):
  kwargs = {}
@@ -205,7 +208,7 @@ def request(self, method, url, headers, post_data=None):
 
  try:
  try:
- result = self._session.request(
+ result = self._thread_local.session.request(
  method,
  url,
  headers=headers,
@@ -285,8 +288,8 @@ def _handle_request_error(self, e):
  raise error.APIConnectionError(msg, should_retry=should_retry)
 
  def close(self):
- if self._session is not None:
- self._session.close()
+ if self._thread_local.session is not None:
+ self._thread_local.session.close()
 
 
 class UrlFetchClient(HTTPClient):