Switch Dockerfile to python 3

[ordinarygulp]

per blog: "We plan on ending mainstream support on 1st April 2019, where upon Python 3.5+ will be the only officially supported platform"
https://matrix.org/blog/2018/12/21/porting-synapse-to-python-3/

[Half-Shot]

(edited title to reflect that this is a docker PR)

[ordinarygulp]

Thanks!

I tested this on my local installation (https://hub.docker.com/r/d2dyno/synapse-python3), and thus far have only encountered one minor notice:
/usr/local/lib/python3.7/site-packages/requests/__init__.py:91: RequestsDependencyWarning: urllib3 (1.25.2) or chardet (3.0.4) doesn't match a supported version! RequestsDependencyWarning)

[anoadragon453]

@d2dyno We're aware of the urllib3 issue and should be unrelated to this PR specifically.

[richvdh]

I don't particularly object to this change, but just so we're all clear:

the PYTHON_VERSION defined in the file just defines the default python version to build with. you can choose whichever version you prefer as documented in the comment at the top of the file.

Note that the default docker images distributed at https://hub.docker.com/r/matrixdotorg/synapse use 3.6.

[richvdh]

Suggested change

	Docker container now builds with Python 3.7.
	Docker image now uses Python 3.7 by default.

[richvdh]

Suggest making this a misc file rather than a feature. It's unlikely to affect the vast majority of regular users.

[codecov]

Codecov Report

Merging #5132 into develop will decrease coverage by 1.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5132      +/-   ##
===========================================
- Coverage    61.69%   60.43%   -1.27%     
===========================================
  Files          336      328       -8     
  Lines        34549    34093     -456     
  Branches      5674     5627      -47     
===========================================
- Hits         21316    20605     -711     
- Misses       11702    12017     +315     
+ Partials      1531     1471      -60

[hawkowl]

Fixed by way of #5546, which sets it to be the default to be built and sent to Docker Hub as well.