Python 3 support

[oprypin]

Catapult relies on Python 2 but it's nearing end-of-life in the year 2020.
This is a tracking bug for Python 3 compatibility fixes.

[ITmaze]

Using a python:latest container, which defaults to python3, the following test fails:

# catapult/tracing/bin/run_py_tests
[50/139] trace_data.trace_data_unittest.TraceDataTest.testSerializeTraceback (most recent call last):
File "/tmp/catapult/tracing/tracing/trace_data/../../bin/trace2html", line 13, in
from tracing_build import trace2html
File "/tmp/catapult/tracing/tracing_build/trace2html.py", line 17, in
from py_vulcanize import generate
File "/tmp/catapult/common/py_vulcanize/py_vulcanize/init.py", line 11, in
from py_vulcanize.project import Project
File "/tmp/catapult/common/py_vulcanize/py_vulcanize/project.py", line 13, in
from py_vulcanize import resource_loader
File "/tmp/catapult/common/py_vulcanize/py_vulcanize/resource_loader.py", line 13, in
from py_vulcanize import html_module
File "/tmp/catapult/common/py_vulcanize/py_vulcanize/html_module.py", line 10, in
from py_vulcanize import parse_html_deps
File "/tmp/catapult/common/py_vulcanize/py_vulcanize/parse_html_deps.py", line 34, in
import bs4
File "/tmp/catapult/third_party/beautifulsoup4/bs4/init.py", line 175
except Exception, e:
^
SyntaxError: invalid syntax
[51/139] trace_data.trace_data_unittest.TraceDataTest.testSerialize failed unexpectedly:
Traceback (most recent call last):
File "/tmp/catapult/tracing/tracing/trace_data/trace_data_unittest.py", line 21, in testSerialize
ri.Serialize(trace_path)
File "/tmp/catapult/tracing/tracing/trace_data/trace_data.py", line 198, in Serialize
subprocess.check_output(cmd)
File "/usr/local/lib/python3.7/subprocess.py", line 389, in check_output
**kwargs).stdout
File "/usr/local/lib/python3.7/subprocess.py", line 481, in run
output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['python', '/tmp/catapult/tracing/tracing/trace_data/../../bin/trace2html', '/tmp/tmpnzrx83ru/tmp_s87kz_4', '--output', '/tmp/tmp7px4mrki/test_trace.json', '--title', '']' returned non-zero exit status 1.
138 tests passed, 0 skipped, 1 failure.

[oprypin]

It works by launching through python3 and only if python is 2.7.

[ITmaze]

# python3 --version
Python 3.7.1

# python2 --version
Python 2.7.13

One point is that the docker container has a symlink called python, that was linked to python3, which caused the failed test. Once I replaced it with a symlink to python2, all worked as expected.