Fixed adding a duplicate column to the alembic table

[leemurus]

Fix problem with new sqlalchemy for alembic migrations.

What changed?

• old sqlalchemy function table.append_column link
• new sqlalchemy function table.append_column link

replace_existing: When True, allows replacing existing
columns. When False, the default, an warning will be raised
if a column with the same .key already exists. A future
version of sqlalchemy will instead rise a warning.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

[coveralls]

coverage: 85.756%. remained the same
when pulling 01ce534 on leemurus:fix/patch-alembic
into a5390b9 on xzkostyan:master.