Build fails: Various erros in py37-tracecontext

[Oberon00]

This happens on master since the merge of #291 and all new PRs. I suspect a dependency update to be the cause.

For example https://travis-ci.org/open-telemetry/opentelemetry-python/jobs/623466280:

py37-tracecontext installed: aiohttp==3.6.2,async-timeout==3.0.1,attrs==19.3.0,certifi==2019.11.28,chardet==3.0.4,Click==7.0,Flask==1.1.1,idna==2.8,itsdangerous==1.1.0,Jinja2==2.10.3,MarkupSafe==1.1.1,multidict==4.7.0,requests==2.22.0,urllib3==1.25.7,Werkzeug==0.16.0,yarl==1.4.2
...

test_multiple_requests_with_illegal_traceparent (__main__.AdvancedTest) ... Segmentation fault (core dumped)
ERROR: InvocationError for command /home/travis/build/open-telemetry/opentelemetry-python/scripts/tracecontext-integration-test.sh (exited with code 139))
Note: this might indicate a fatal error signal (139 - 128 = 11: SIGSEGV) (exited with code 139)

Or https://travis-ci.org/open-telemetry/opentelemetry-python/jobs/623458269, where every test fails with:

py37-tracecontext installed: aiohttp==3.6.2,async-timeout==3.0.1,attrs==19.3.0,certifi==2019.11.28,chardet==3.0.4,Click==7.0,Flask==1.1.1,idna==2.8,itsdangerous==1.1.0,Jinja2==2.10.3,MarkupSafe==1.1.1,multidict==4.7.0,requests==2.22.0,urllib3==1.25.7,Werkzeug==0.16.0,yarl==1.4.2
...

test_traceparent_version_0xff (__main__.TraceContextTest) ... Error handling request
Traceback (most recent call last):
  File "/home/travis/build/open-telemetry/opentelemetry-python/.tox/py37-tracecontext/lib/python3.7/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/home/travis/build/open-telemetry/opentelemetry-python/.tox/py37-tracecontext/lib/python3.7/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/home/travis/build/open-telemetry/opentelemetry-python/target/trace-context/test/server.py", line 53, in scope_handler
    async with ClientSession(headers = headers, timeout = self.timeout) as session:
  File "/home/travis/build/open-telemetry/opentelemetry-python/.tox/py37-tracecontext/lib/python3.7/site-packages/aiohttp/client.py", line 268, in __init__
    real_headers = CIMultiDict(headers)  # type: CIMultiDict[str]
TypeError: CIMultiDict keys should be either str or subclasses of str
Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x7efcde7e8668>
ERROR

[Oberon00]

The only thing that changed from the last successful build is multidict 4.6.1 -> 4.7.0, so that is probably the cause.

[Oberon00]

This should probably be reported to upstream https://github.com/w3c/trace-context
EDIT: Done: w3c/trace-context#366

[condorcet]

It was bug in multidict library aio-libs/multidict#418 There is no such problem since 4.7.1.