Spec types for all SkillEntry properties

[NeonDaniel]

Add SkillEntry property return type annotations and ensure default values match expected return type
Needs rebase on #81
Closes #47

[codecov]

Codecov Report

Merging #85 (5c3f529) into master (ba89385) will increase coverage by 1.71%.
The diff coverage is 84.00%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   56.38%   58.09%   +1.71%     
==========================================
  Files          33       34       +1     
  Lines        2680     2718      +38     
==========================================
+ Hits         1511     1579      +68     
+ Misses       1169     1139      -30     

Impacted Files	Coverage Δ
ovos_skills_manager/local_skill/__init__.py	70.14% <70.14%> (ø)
ovos_skills_manager/appstores/local.py	65.51% <78.57%> (+37.31%)	⬆️
ovos_skills_manager/skill_entry.py	67.23% <87.50%> (+0.87%)	⬆️
ovos_skills_manager/github/api.py	32.13% <90.90%> (+0.14%)	⬆️
ovos_skills_manager/github/__init__.py	69.44% <100.00%> (+0.74%)	⬆️
ovos_skills_manager/github/raw.py	63.89% <100.00%> (+0.10%)	⬆️
ovos_skills_manager/github/utils.py	94.01% <100.00%> (ø)
ovos_skills_manager/requirements.py	30.43% <100.00%> (+7.24%)	⬆️
...os_skills_manager/appstores/mycroft_marketplace.py	56.89% <0.00%> (-3.45%)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6594141...5c3f529. Read the comment docs.