Python version deprecations and add 3.8 testing

[jantman]

This PR fixes a number of issues related to python versions:

• 438 - Announce deprecation of Python 2.7 support on January 1, 2020

• 437 - Announce deprecation of Python 3.4 support on January 1, 2020

• 436 - Begin testing under Python 3.8 and build Docker image off of Python 3.8

See the Changelog entry for details.

[codecov-io]

Codecov Report

Merging #444 into develop will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           develop   #444   +/-   ##
======================================
  Coverage      100%   100%           
======================================
  Files           36     36           
  Lines         2677   2696   +19     
  Branches       410    410           
======================================
+ Hits          2677   2696   +19

Impacted Files	Coverage Δ
awslimitchecker/checker.py	100% <100%> (ø)	:arrow_up:
awslimitchecker/services/ec2.py	100% <0%> (ø)	:arrow_up:
awslimitchecker/metrics/base.py	100% <0%> (ø)	:arrow_up:
awslimitchecker/alerts/base.py	100% <0%> (ø)	:arrow_up:
awslimitchecker/services/base.py	100% <0%> (ø)	:arrow_up:
awslimitchecker/version.py	100% <0%> (ø)	:arrow_up:
awslimitchecker/limit.py	100% <0%> (ø)	:arrow_up:

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