requests: Improvements for requests integration (#573)

Adding a TestBase class which wraps a tracer provider that is configured with a memory span
exporter.  This class inherits from unitest.TestCase, hence other test classes
can inherit from it to get access to the underlying memory span exporter and
tracer provider.

Adding a mock propagator that could be used for testing propagation in different packages.
It was implemented in the opentracing-shim and this commit moves it to a generic
place.

Adding disable_session(), which can be used to disable the instrumentation on a single
requests' session object.

tox.ini

@@ -167,9 +167,13 @@ commands_pre =
   psycopg2: pip install {toxinidir}/ext/opentelemetry-ext-dbapi
   psycopg2: pip install {toxinidir}/ext/opentelemetry-ext-psycopg2
   http-requests: pip install {toxinidir}/ext/opentelemetry-ext-http-requests
+  http-requests: pip install {toxinidir}/tests/util
+  http-requests: pip install {toxinidir}/opentelemetry-sdk
+  http-requests: pip install {toxinidir}/ext/opentelemetry-ext-http-requests[test]
   jaeger: pip install {toxinidir}/opentelemetry-sdk
   jaeger: pip install {toxinidir}/ext/opentelemetry-ext-jaeger
   opentracing-shim: pip install {toxinidir}/opentelemetry-sdk {toxinidir}/ext/opentelemetry-ext-opentracing-shim
+  opentracing-shim: pip install {toxinidir}/tests/util
   zipkin: pip install {toxinidir}/opentelemetry-sdk
   zipkin: pip install {toxinidir}/ext/opentelemetry-ext-zipkin
 
@@ -205,6 +209,7 @@ deps =
   black
   psutil
   readme_renderer
+  httpretty
 
 commands_pre =
   python scripts/eachdist.py install --editable