[wip] Alternative API: allow to access DB explicitly

[kmike]

TODO:

• [x] DB object
• [ ] allow to inject DB to callbacks
• [ ] tests
• [x] docs
• [ ] check if old Pythons need to be dropped

[codecov-io]

Codecov Report

Merging #2 into master will decrease coverage by 6.84%. The diff coverage is 84.84%.

@@             Coverage Diff             @@
##            master       #2      +/-   ##
===========================================
- Coverage   100.00%   93.15%   -6.85%     
===========================================
  Files            2        3       +1     
  Lines           53       73      +20     
  Branches         5        5              
===========================================
+ Hits            53       68      +15     
- Misses           0        5       +5     

Impacted Files	Coverage Δ
scrapy_crawl_once/db.py	80.00% <80.00%> (ø)
scrapy_crawl_once/__init__.py	100.00% <100.00%> (ø)
scrapy_crawl_once/middlewares.py	100.00% <100.00%> (ø)

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