Modal

- Fixed a bug introduced in 0.63.0 that broke `modal.Cls.with_options`

- Adds warning about future deprecation of `retries` for generators. Retries are being deprecated as they can lead to nondetermistic generator behavior.

- Fixed a bug in `Volume.copy_files()` where some source paths may be ignored if passed as `bytes`.
- `Volume.read_file`, `Volume.read_file_into_fileobj`, `Volume.remove_file`, and `Volume.copy_files` can no longer take both string or bytes for their paths. They now only accept `str`.

* Fixes issue with `Cls.lookup` not working (at all) after upgrading to v0.63.0. **Note**: this doesn't fix the cross-version lookup incompatibility introduced in 0.63.0.

* Changes how containers are associated with methods of `app.cls()`-decorated Modal "classes".

Previously each `method` and web endpoint of a class would get its own set of isolated containers and never run in the same container as other sibling methods.
Starting in this version, all `methods` and web endpoints will be part of the same container pool. Notably, this means all methods will scale up/down together, and options like `keep_warm` and `concurrency_limit` will affect the total number of containers for all methods in the class combined, rather than individually.

**Version incompatibility warning:** Older clients (below 0.63) can't use classes deployed by new clients (0.63 and above), and vice versa. Apps or standalone clients using `Cls.lookup(...)` to invoke Modal classes need to be upgraded to version `0.63` at the same time as the deployed app that's being called into.

* `keep_warm` for classes is now an attribute of the `app.cls()` decorator rather than individual methods.