AVRO-2765: Py3 Support: Use open(), not file()

[kojiromike]

Jira

• My PR addresses AVRO-2765
• It references it in the title.
• No dependencies.

Tests

• Needs a regression test

Commits

• All reference Jira issues in their subject lines.
• My commits follow the guidelines from "How to write a good git commit message".

Documentation

• No feature changes, no additional documentation needed.

[Fokko]

Sad CI, we're unable to import the ContextManager module:

+ ./build.sh lint test
GLOB sdist-make: /home/travis/build/apache/avro/lang/py/setup.py
lint create: /home/travis/build/apache/avro/lang/py/.tox/lint
ERROR: invocation failed (exit code 1), logfile: /home/travis/build/apache/avro/lang/py/.tox/lint/log/lint-0.log
================================== log start ===================================
ImportError: cannot import name 'ContextManager'
=================================== log end ====================================
ERROR: InvocationError for command /usr/bin/python3 -m virtualenv --no-download --python /usr/bin/python3 lint (exited with code 1)
___________________________________ summary ____________________________________
ERROR:   lint: InvocationError for command /usr/bin/python3 -m virtualenv --no-download --python /usr/bin/python3 lint (exited with code 1)
Total Elapsed time:  21m 22s

[kojiromike]

Huh, I haven't been able to reproduce that problem locally. Hopefully it was just a fluke.

[Fokko]

Thanks again @kojiromike