chore: use poetry as dependency manager (#242)

* Adopts [Poetry](https://python-poetry.org/) as a dependency manager
* Introduces dependency version requirements
* Deprecates Python 3.7 support

**TODO**
- [x] Update developer guide
- [x] Add back `playwright`, `manifest-ml`, and `jupyter` to dependency
group

**Not Doing => Fast Follow**
- Investigate single source for version, perhaps relying on GitHub tags
and [tackling this
issue](https://github.com/hwchase17/langchain/issues/26)