Extension at cli, -e jinja2='jinja2_python_version.PythonVersionExtension'

[chfw]

Before raising the PR, here is a check list:

• have you written unit tests for your code changes?
• have you updated the change log?
• can someone else understand your changes without your explanation?
• are you proud of your code changes?

fix #333

[codecov-io]

Codecov Report

Merging #332 into dev will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #332      +/-   ##
==========================================
+ Coverage   95.37%   95.58%   +0.21%     
==========================================
  Files          51       51              
  Lines        2553     2584      +31     
==========================================
+ Hits         2435     2470      +35     
+ Misses        118      114       -4

Impacted Files	Coverage Δ
moban/main.py	99.2% <100%> (+0.06%)	⬆️
moban/core/moban_factory.py	92.76% <100%> (+3.17%)	⬆️
...sts/integration_tests/test_command_line_options.py	100% <100%> (ø)	⬆️
moban/jinja2/engine.py	98.8% <100%> (+0.05%)	⬆️
moban/constants.py	100% <100%> (ø)	⬆️

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