Introduce BaseReporter type

[yurishkuro]

Turns out we don't have a good type to use in type declarations in #328.

[codecov]

Codecov Report

Merging #329 (82207fb) into master (948b37e) will increase coverage by 0.08%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   95.43%   95.51%   +0.08%     
==========================================
  Files          25       25              
  Lines        1971     2030      +59     
  Branches      273      274       +1     
==========================================
+ Hits         1881     1939      +58     
- Misses         56       57       +1     
  Partials       34       34              

Impacted Files	Coverage Δ
jaeger_client/reporter.py	92.72% <88.88%> (-0.32%)	⬇️
jaeger_client/rate_limiter.py	100.00% <0.00%> (ø)
jaeger_client/span_context.py	100.00% <0.00%> (ø)
jaeger_client/span.py	96.66% <0.00%> (+0.11%)	⬆️
jaeger_client/utils.py	90.32% <0.00%> (+0.15%)	⬆️
crossdock/server/endtoend.py	96.66% <0.00%> (+0.23%)	⬆️
jaeger_client/config.py	96.19% <0.00%> (+0.58%)	⬆️
crossdock/server/server.py	88.40% <0.00%> (+0.80%)	⬆️

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