Potential dependency conflicts between fan and opentracing

[NeolithEra]

Hi, as shown in the following full dependency graph of fan, fan requires opentracing (the latest version)， while the installed version of basictracer(3.1.0) requires opentracing>=2.0,<3.0.

According to Pip's “first found wins” installation strategy, opentracing 2.2.0 is the actually installed version.

Although the first found package version opentracing 2.2.0 satisfies the later dependency constraint （opentracing>=2.0,<3.0), it will lead to a build failure once developers release a version of opentracing whose version number is greater than 3.0.

It'll be good if we just adjust the constraints (if possible) here, which can avoid potential dependency conflicts in the near future.

Dependency tree--------

fan<version range:>
| +-aiohttp<version range:>
| +-aiozk<version range:>=0.15>
| +-basictracer<version range:>
| | +-opentracing<version range:>=2.0,<3.0>
| | +-protobuf<version range:>=3.0.0b2.post2>
| | +-six<version range:>=1.10.0,<2.0>
| +-kazoo<version range:>
| +-opentracing<version range:>
| +-py-zipkin<version range:<1.0.0,>=0.18.0>
| +-requests<version range:>
| +-sanic<version range:>
| +-tipsi-tools<version range:>=1.49.0>
| | +-python-json-logger<version range:>=0.1.5>
| | +-pyyaml<version range:>=3.12>```

Thanks for your attention.
Best,
Neolith

[NeolithEra]

Suggestion:

Fix your direct dependencies to be opentracing>=2.0,<3.0 and basictracer==3.1.0, to remove this conflict.
I have checked this revision will not affect your downstream projects now.

@cybergrind May I pull a request to fix this issue?

[NeolithEra]

@cybergrind Could you help me review this issue?