fix: add concurrency docs

Author: untitaker

README.md

@@ -88,6 +88,15 @@ breadcrumbs manually:
         "message": "hi"
     })
 
+### Concurrency
+
+* Sentry-Python currently does not support gevent-based setups.
+* On ``init``, Sentry-Python spawns a thread on its own. That means if you use
+  ``uwsgi``, you currently need to enable threads.
+* On Python 3.7, Sentry-Python supports and automatically uses ``ContextVars``.
+  This should effectively enable Sentry-Python to work with ``asyncio`` under
+  that Python version.
+
 ## Integrations
 
 Head over to [the other pages](./docs/) to check out integrations, which use

sentry_sdk/utils.py

@@ -372,8 +372,6 @@ def to_string(value):
 except ImportError:
     from threading import local
 
-    _nothing = object()
-
     class ContextVar(object):
         # Super-limited impl of ContextVar