* add support for variable string sizes in ColumnSchema

* create test workflow

* add install dependencies step

* install wheel in dependency step

* add wheel to tests dependencies and remove step from workflow

* fix syntax error

* move wheel from tests_require to install_requires array

- When creating an interleaved table, backwards migrations are broken because
 of this error: `spanner_orm.error.SpannerError: Multiple classes match
 table_interaction_evidence_model, add more specificity`. This happens because
 schemas are fetched using data from Spanner, which leads to multiple tables
 being registered with the same name, causing conflict when an interleaved
 table's migration is validated.

Solution: When fetching from Spanner, re-register model classes so there's no
conflict.

- This doesn't require a constraint (its the equivalent of a
 reverse relationship in sqlalchemy / django). Ideally the fix
 can be a general purpose reverse relationship manager, but
 that's lot of work. This unblocks the use of relationship for
 the simple case of interleaved parent-child tables.

…possible (maroux#26)

Migrations should use schema definition frozen in time at the time of creation so that updating models code doesn't automatically affect a historical migration.

- Also check black formatting in tests

- This allows use of several [Spanner functions](https://cloud.google.com/spanner/docs/functions-and-operators) on existing columns