Not able to build Heron in Centos 7 image due to Python version

[nicknezis]

Describe the bug
When building in Centos 7 Docker image, the build fails due to uvloop Python library depending on Python 3.7 which is newer than the Centos 7 provided Python 3.6.8.

To Reproduce
Steps to reproduce the behavior:
When building using ./docker/scripts/build-artifacts.sh centos7 0.20 /tmp/ the build fails with the following output message.

ERROR: /heron/heron/tools/ui/src/python/BUILD:24:11: PexPython heron/tools/ui/src/python/heron-ui.pex failed (Exit 1): pex_wrapper.pex failed: error executing command 
  (cd /root/.cache/bazel/_bazel_root/f4ab758bd53020512013f7dfa13b6902/execroot/org_apache_heron && \
  exec env - \
    PATH=/bin:/usr/bin:/usr/local/bin \
    PEX_ROOT=.pex \
    PEX_VERBOSE=0 \
  bazel-out/host/bin/tools/rules/pex/pex_wrapper.pex --not-zip-safe --pex-root .pex --entry-point heron.tools.ui.src.python.main --output-file bazel-out/k8-fastbuild/bin/heron/tools/ui/src/python/heron-ui.pex --disable-cache --python-shebang '#!/usr/bin/env python3' --no-compile bazel-out/k8-fastbuild/bin/heron/tools/ui/src/python/heron-ui.pex_manifest)
Execution platform: @local_config_platform//:host
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3.6 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-download-315rix09/uvloop/setup.py'"'"'; __file__='"'"'/tmp/pip-download-315rix09/uvloop/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' --no-user-cfg egg_info --egg-base /tmp/pip-download-315rix09/uvloop/pip-egg-info
         cwd: /tmp/pip-download-315rix09/uvloop/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-download-315rix09/uvloop/setup.py", line 5, in <module>
        raise RuntimeError('uvloop requires Python 3.7 or greater')
    RuntimeError: uvloop requires Python 3.7 or greater
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
pid: 11744 -> /usr/bin/python3.6 /root/.cache/bazel/_bazel_root/f4ab758bd53020512013f7dfa13b6902/execroot/org_apache_heron/.pex/pip.pex/49a647f5ed9e81dc8f85560600c53fbb4dfc60f0 --disable-pip-version-check --isolated --no-python-version-warning -q --no-cache-dir download --dest /tmp/tmpz84g06vr/resolved_dists/cp36-cp36m protobuf==3.8.0 setuptools==46.1.3 colorlog==2.6.1 PyYAML==3.13 requests==2.12.3 click==7.1.2 fastapi==0.60.1 jinja2==2.11.2 aiofiles==0.5.0 uvicorn==0.11.7 raised Executing /usr/bin/python3.6 /root/.cache/bazel/_bazel_root/f4ab758bd53020512013f7dfa13b6902/execroot/org_apache_heron/.pex/pip.pex/49a647f5ed9e81dc8f85560600c53fbb4dfc60f0 --disable-pip-version-check --isolated --no-python-version-warning -q --no-cache-dir download --dest /tmp/tmpz84g06vr/resolved_dists/cp36-cp36m protobuf==3.8.0 setuptools==46.1.3 colorlog==2.6.1 PyYAML==3.13 requests==2.12.3 click==7.1.2 fastapi==0.60.1 jinja2==2.11.2 aiofiles==0.5.0 uvicorn==0.11.7 failed with 1
INFO: Elapsed time: 365.446s, Critical Path: 80.42s
INFO: 1030 processes: 211 internal, 819 local.
FAILED: Build did NOT complete successfully

Expected behavior
The build should produce artifacts without error. Also the bazel_configure.py version check file should properly match the dependency if we actually need Python 3.7 going forward.

Operating System

• OS: Centos 7
• Version master branch

Additional context
N/A

[nicknezis]

This uvicorn update addresses the breaking change described here: encode/uvicorn#952

[thinker0]

Thanks.
Now the CeontOS7 build is successful.