fix: pin uvicorn for python 3.6

[bojiang]

Description

Motivation and Context

encode/uvicorn#1261

How Has This Been Tested?

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[codecov]

Codecov Report

Merging #2180 (0a7815a) into main (5ab95a6) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2180      +/-   ##
==========================================
- Coverage   45.90%   45.83%   -0.08%     
==========================================
  Files          93       94       +1     
  Lines        7877     7846      -31     
==========================================
- Hits         3616     3596      -20     
+ Misses       4261     4250      -11     

Impacted Files	Coverage Δ
bentoml/_internal/utils/lazy_loader.py	52.94% <0.00%> (-29.42%)	⬇️
bentoml/_internal/io_descriptors/image.py	61.01% <0.00%> (-1.89%)	⬇️
bentoml/_internal/io_descriptors/numpy.py	58.73% <0.00%> (-1.59%)	⬇️
bentoml/_internal/io_descriptors/pandas.py	55.17% <0.00%> (-0.51%)	⬇️
bentoml/_internal/cli/yatai.py	0.00% <0.00%> (ø)
bentoml/_internal/runner/remote.py	0.00% <0.00%> (ø)
bentoml/_internal/runner/container.py	0.00% <0.00%> (ø)
bentoml/_internal/server/runner_app.py	0.00% <0.00%> (ø)
bentoml/_internal/cli/bento_management.py	0.00% <0.00%> (ø)
bentoml/_internal/cli/model_management.py	0.00% <0.00%> (ø)
... and 7 more