Make db requirements optional

[simonhkswan]

Now you can install the requirements for connecting to a DB optionally. e.g.

pip install 'insight[db]'

[codecov[bot]]

Codecov Report

Merging #110 (ab0d365) into master (a318a2e) will decrease coverage by 0.9%. The diff coverage is 81.9%.

@@           Coverage Diff            @@
##           master    #110     +/-   ##
========================================
- Coverage    80.1%   79.3%   -0.9%     
========================================
  Files          11      11             
  Lines         850     870     +20     
  Branches      156     130     -26     
========================================
+ Hits          681     690      +9     
- Misses        130     140     +10     
- Partials       39      40      +1     

Impacted Files	Coverage Δ
src/insight/database/utils.py	75.3% <20.0%> (-6.3%)	:arrow_down:
src/insight/metrics/base.py	75.0% <45.4%> (-3.6%)	:arrow_down:
src/insight/plot.py	68.6% <89.6%> (ø)
src/insight/__init__.py	100.0% <100.0%> (ø)
src/insight/database/schema.py	100.0% <100.0%> (ø)

[sonarcloud[bot]]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

61.5% Coverage
0.0% Duplication