🔨 Code refactoring

[CLiu13]

Related to #146

Before raising the PR, here is a check list:

1. have you written unit tests for your code changes?
2. have you run "make format"?
3. are you requesting to "dev"?
4. have you updated the change log?
5. do you think that you can understand your changes after 6 month?
   5.1) can someone else understand your changes without your explanation?
6. are you pround of your code changes?
   6.1) do you have the feeling of achievement?
7. please add your name and github link to contributors.rst in alphabetical order.

[codecov-io]

Codecov Report

Merging #147 into dev will increase coverage by 0.4%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##              dev     #147     +/-   ##
=========================================
+ Coverage   97.03%   97.44%   +0.4%     
=========================================
  Files          40       40             
  Lines        1925     1916      -9     
=========================================
- Hits         1868     1867      -1     
+ Misses         57       49      -8

Impacted Files	Coverage Δ
moban/plugins.py	96.5% <88.23%> (+5.05%)	⬆️

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[chfw]

Good to go!