Update PyPI classifiers

[mauriciovasquezbernal]

• Update development status to beta
• Add Python 3.8 to list of languages

[codecov-io]

Codecov Report

Merging #525 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   89.47%   89.37%   -0.10%     
==========================================
  Files          43       43              
  Lines        2213     2193      -20     
  Branches      250      250              
==========================================
- Hits         1980     1960      -20     
  Misses        161      161              
  Partials       72       72              

Impacted Files	Coverage Δ
...ry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py	67.56% <0.00%> (-0.58%)	⬇️
...lemetry-sdk/src/opentelemetry/sdk/util/__init__.py	85.54% <0.00%> (-0.35%)	⬇️
...ts/src/opentelemetry/ext/http_requests/__init__.py	89.18% <0.00%> (-0.29%)	⬇️
...src/opentelemetry/ext/opentracing_shim/__init__.py	95.86% <0.00%> (-0.17%)	⬇️
...xt-jaeger/src/opentelemetry/ext/jaeger/__init__.py	86.39% <0.00%> (-0.16%)	⬇️
...emetry-sdk/src/opentelemetry/sdk/trace/__init__.py	92.46% <0.00%> (-0.06%)	⬇️
...etry-sdk/src/opentelemetry/sdk/metrics/__init__.py	94.84% <0.00%> (-0.03%)	⬇️
.../src/opentelemetry/sdk/metrics/export/aggregate.py	100.00% <0.00%> (ø)
...app/src/opentelemetry_example_app/flask_example.py	100.00% <0.00%> (ø)

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