Add tests using symbol_version as library

[ansasaki]

The added tests use the module as a library instead as a command line
application.

The code was reorganized, adding huge comments separators.

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by 1.72%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   90.12%   91.85%   +1.72%     
==========================================
  Files           2        2              
  Lines         699      712      +13     
  Branches      177      181       +4     
==========================================
+ Hits          630      654      +24     
+ Misses         36       30       -6     
+ Partials       33       28       -5

Impacted Files	Coverage Δ
src/symbol_version/symbol_version.py	91.8% <94.11%> (+1.73%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 52

• 85 of 85 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 93.023%

---

Totals
Change from base Build 48:	1.2%
Covered Lines:	682
Relevant Lines:	712

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 52

• 85 of 85 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 93.023%

---

Totals
Change from base Build 48:	1.2%
Covered Lines:	682
Relevant Lines:	712

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 52

• 85 of 85 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 93.023%

---

Totals
Change from base Build 48:	1.2%
Covered Lines:	682
Relevant Lines:	712

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💛 - Coveralls