Error Handling in rewrite is incorrect.

[theGOTOguy]

Look at:
https://github.com/theskumar/python-dotenv/blob/master/src/dotenv/main.py#L136

If lines 136, 137 or 140 there are ever hit, an error "local variable 'dest' referenced before assignment" will be thrown, because the dest variable only exists within the scope of the with block above.

[theGOTOguy]

Resolved by #414

[theskumar]

I had to revert #414 because the test was failing. Do you want to take another stab at it?

[theGOTOguy]

Looks like it was just a linter error. I submitted a new PR -- #423 should work.