Relax ruamel.yaml dependancy cap and support python 3.7

[ThomasHickman]

Following on from common-workflow-language/schema_salad#189 and fixes #822

[codecov]

Codecov Report

Merging #831 into master will increase coverage by 4.97%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage   67.12%   72.09%   +4.97%     
==========================================
  Files          30       34       +4     
  Lines        5582     6154     +572     
  Branches     1411     1564     +153     
==========================================
+ Hits         3747     4437     +690     
+ Misses       1431     1282     -149     
- Partials      404      435      +31

Impacted Files	Coverage Δ
cwltool/docker_id.py	48.97% <0%> (-2.09%)	⬇️
cwltool/context.py	96.42% <0%> (-0.87%)	⬇️
cwltool/main.py	35.78% <0%> (ø)
cwltool/flatten.py	78.94% <0%> (ø)
cwltool/utils.py	47.48% <0%> (ø)
cwltool/__init__.py	100% <0%> (ø)
cwltool/software_requirements.py	29.72% <0%> (+1.35%)	⬆️
cwltool/load_tool.py	82.67% <0%> (+1.48%)	⬆️
cwltool/argparser.py	83.73% <0%> (+2.39%)	⬆️
cwltool/job.py	63.15% <0%> (+3.4%)	⬆️
... and 12 more

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[mr-c]

lxml has wheels for manylinux1 & py37 but not yet windows: https://pypi.org/project/lxml/#files

https://bugs.launchpad.net/lxml/+bug/1757938

https://mailman-mail5.webfaction.com/pipermail/lxml/2018-July/023964.html

[ThomasHickman]

Note: I added an issue to typings for the python 3.7 typing bug we are finding: python/typing#573. Their solution is to not require typing for python 3.5+, as it is built in (which is fair enough). I think we should implement this

[ThomasHickman]

^ changed that to basically typing>=3.5.3; python_version < "3.7" as python 3.7 is the first release where typing is not released as "provisional"

[mr-c]

@ThomasHickman Thanks!