First patch and more specific dependencies

## Fixes

- Set explicit `stacklevel` when warning about schema-wide `many` setting depending on initialization state. This should help users see the exact line they wrote that triggered the warning.

## Misc

- Restrict main dependency `marshmallow>=3.12.0`; pin all development dependencies to specific versions
- Fix minor cosmetic issues in project config
- Rework and refactor development scripts

First stable release