Fixed DBError on close, converted to use Pipenv and few other updates

[ouahibelhanchi]

• Converted to use Pipenv
• Attempt to close database handle only if the database has been opened
• Renamed variables to avoid shadowing built-in functions (e.g: dir)
• Catching exception DBError instead of the too general exception Exception
• Defined database attribute inside init

[codecov]

Codecov Report

Merging #24 into master will decrease coverage by 2.41%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage    91.3%   88.88%   -2.42%     
==========================================
  Files           2        2              
  Lines          69       72       +3     
  Branches        9       10       +1     
==========================================
+ Hits           63       64       +1     
- Misses          3        4       +1     
- Partials        3        4       +1

Impacted Files	Coverage Δ
scrapy_deltafetch/middleware.py	88.57% <87.5%> (-2.48%)	⬇️

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