Remove support of EOL 3.4 add in 3.8

[kddejong]

Issue #, if available:

Description of changes:

• Remove support for Python 3.4.
• Add in support for Python 3.8

PyYAML a dependent package has removed support.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kddejong]

Current download proportion rates for Python 3.4 are less than 1%. They will have to pin an older version to continue to download.

@PatMyron maybe we pin pyyaml once to a version supported by Python 3.4 Release. Then EOL.
That way users can have a version that will work until they can migrate.

[codecov]

Codecov Report

Merging #1289 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1289      +/-   ##
==========================================
+ Coverage    87.8%   87.81%   +<.01%     
==========================================
  Files         137      137              
  Lines        8104     8122      +18     
  Branches     1990     1991       +1     
==========================================
+ Hits         7116     7132      +16     
- Misses        584      586       +2     
  Partials      404      404

Impacted Files	Coverage Δ
src/cfnlint/helpers.py	83.83% <0%> (-1.2%)	⬇️
src/cfnlint/decode/cfn_json.py	72.33% <0%> (-0.22%)	⬇️
src/cfnlint/transform.py	90.9% <0%> (+0.11%)	⬆️
src/cfnlint/rules/__init__.py	88.8% <0%> (+0.13%)	⬆️
src/cfnlint/config.py	85.71% <0%> (+0.76%)	⬆️

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