fix: fix select.join

[Net-Mist]

We discover than requests like select(table1).join(table2, table2.id == table1.table2Id).where([...])
didn't work with clickhouse-sqlalchemy but did work with standard sqlalchemy because select(table1).join(table2, table2.id == table1.table2Id) is a Select object in sqlalchemy2.

This PR fix this (just changed the return of the join method) and add a test to prove the fix works.

The diff in the test is massive because I added a for loop to test both ways to compute the join statement. Let me know if you think there is a better way to write the test

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: 85.897% (+0.07%) from 85.824%
when pulling a71f687 on Net-Mist:fix_join
into 46482bf on xzkostyan:master.

[Net-Mist]

Hello @xzkostyan
Now that the other PR has been merged, the unit-tests for this one are all green. Let me know if you think I should change something here