Pin contextvars to latest version 2.4 (#63)

Author: pyup-bot

requirements_dev.txt

@@ -5,7 +5,7 @@ watchdog==0.9.0
 Sphinx==2.3.1
 twine==3.1.1
 
-contextvars;python_version<"3.7"
+contextvars==2.4;python_version<"3.7"
 fastapi==0.42.0
 jaeger-client>=4.1.0
 opentracing>=2.2.0