Add session variable to DBSession init

[teamhide]

While using db.session on pycharm, it notify Unresolved attribute reference 'session' for class 'DBSession' errors.

To avoid it, added self.session to DBSession.

[codecov-commenter]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           60        61    +1     
=========================================
+ Hits            60        61    +1     

Impacted Files	Coverage Δ
fastapi_sqlalchemy/middleware.py	100.00% <100.00%> (ø)

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[mfreeborn]

Thanks for the PR. I don't use Pycharm myself. Happy to resolve this problem. Could I ask you to try one alternative and tell me if this fixes it? Instead of adding self.session = None, what happens if you change line 82 of middleware.py from db: DBSession = DBSession to db: DBSessionMeta = DBSession? Does that fix it or not?

[teamhide]

Works perfectly. I will re-pr for this issue.