fix transaction on DB-API 2.0

[koxudaxi]

The PR fixes transaction on DB-API 2.0.
The Client begins a transaction when calling cursor().

[codecov]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #21   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         397    399    +2     
  Branches       47     48    +1     
=====================================
+ Hits          397    399    +2

Impacted Files	Coverage Δ
pydataapi/dbapi.py	100% <100%> (ø)	⬆️

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