Keep the unique flag in DropIndexOp

[iurisilvio]

Description

Fix #1371

Keep the unique flag in the index generated by DropIndexOp.from_index(...).

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • please include the issue number, and create an issue if none exists, which
    must include a complete example of the issue. one line code fixes without an
    issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• A new feature implementation
  • please include the issue number, and create an issue if none exists, which must
    include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!

[sqla-tester]

OK, this is sqla-tester setting up my work on behalf of CaselIT to try to get revision 5d6616c of this pull request into gerrit so we can run tests and reviews and stuff

[sqla-tester]

New Gerrit review created for change 5d6616c: https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4995

[sqla-tester]

OK, this is sqla-tester setting up my work on behalf of CaselIT to try to get revision 515b4ed of this pull request into gerrit so we can run tests and reviews and stuff

[sqla-tester]

Patchset 515b4ed added to existing Gerrit review https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4995

[CaselIT]

you can add "pull request courtesy of you" if you like

[sqla-tester]

Gerrit review https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4995 has been merged. Congratulations! :)