Provide py38-py311 and py312 compatible way for subclassing

[ap--]

When #152 is merged, user subclasses that override upath.core._FSSpecAccessor will very likely not work correctly with Python3.12, because the 3.12 implementation doesn't use the accessor.

There should be a recommended way for users to subclass UPath for Python3.12 AND Python3.8-Python3.11 without having to maintain two code paths.

Searching GitHub, I could collect an example set of user subclasses that can be split into 3 4 intended use cases:

(1) customize fsspec filesystem creation

here the user wants to change the way the fsspec filesystem instance is created:

• GitHubAccessor which gets token kwarg from environment: https://github.com/juftin/textual-universal-directorytree/blob/110770f2ee40ab5afff7eade635caad644d80848/textual_universal_directorytree/alternate_paths.py#L15-L27
• HTTPAccessor that wraps the fsspec filesystem with a cache: https://github.com/joouha/euporie/blob/620ce11442e5f7d189966950afa3c088b5d0a41f/euporie/core/path.py#L59-L77
• AirflowCloudAccessor which completely customizes fsspec instance creation: https://github.com/apache/airflow/blob/07fd3646fa765859a152731bf135ea944d0753d7/airflow/io/path.py#L46-L68

upath.UPath should offer a classmethod _make_fs() (?) to allow fsspec filesystem instance customization.

It should be possible too to support old code on python3.12 by detecting when the _default_accessor attribute is present and use that to make a compatible _make_fs() (?) fallback method.

(2) customize path formatting

here the user wants to change the way the path is formatted. Examples:

• GitHubAccessor which removes the leading slash: https://github.com/phil65/mknodes/blob/0cd6671fe05d66ad44bd17e5287ac8534eccea61/mknodes/utils/pathhelpers.py#L22-L27
• DataAccessor which returns the full URI: https://github.com/joouha/euporie/blob/620ce11442e5f7d189966950afa3c088b5d0a41f/euporie/core/path.py#L110-L113

(3) fix behavior of some accessor methods

here the behavior of a fsspec filesystem method is fixed by fixing the accessor method. Example:

• BoxAccessor wich fixes an issue with boxfs.mkdir: https://github.com/IBM/boxfs/blob/718fb0071d20a7004f44fe2fa0eac26dc9c3d5d5/src/boxfs/_upath.py#L4-L12

(4) customize storage_options extraction

Not really specific to the Accessor subclassing, because the implementation is overriding __new__ in the UPath subclass. The intention is to extract additional fsspec storage_options from the provided args when instantiating UPath(). Example:

• ObjectStoragePath extracts a connection id from the urlpath: https://github.com/apache/airflow/blob/07fd3646fa765859a152731bf135ea944d0753d7/airflow/io/path.py#L96-L153

TODO

While this list in not complete, it does cover 3 4 reasonably easy to solve cases for making the transition to 3.12 less bumpy for custom subclasses.

• (ALL) detect custom _accessor in py3.12 classes and provide warning with instructions on how to migrate.
• (1) provide customizable fsspec filesystem factory classmethod
• (2) provide instructions for how to migrate to .path (with potential customization)
• (3) recommend overriding UPath.mkdir, etc... directly.
• (4) provide customizable fsspec storage_option extraction classmethod