Exchangelib

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- Add support for reading and writing `PermissionSet` field on folders.
- Add support for Exchange 2019 build IDs.

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- Add `Protocol.expand_dl()` to get members of a distribution list.

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- Lower the session pool size automatically in response to ErrorServerBusy and ErrorTooManyObjectsOpened errors from the
server.
- Unusual slicing and indexing (e.g. `inbox.all()[9000]` and `inbox.all()[9000:9001]`)
is now efficient.
- Downloading large attachments is now more memory-efficient. We can now stream the file content without ever storing
the full file content in memory, using the new
`Attachment.fp` context manager.

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- Add a MAINFEST.in to ensure the LICENSE file gets included + CHANGELOG.md and README.md to sdist tarball
- Renamed `Item.item_id`, `Folder.folder_id` and `Occurrence.item_id` to just
`Item.id`, `Folder.id` and `Occurrence.id`, respectively. This removes redundancy in the naming and provides
consistency. For all classes that have an ID, the ID can now be accessed using the `id` attribute. Backwards
compatibility and deprecation warnings were added.
- Support folder traversal without creating a full cache of the folder hierarchy first, using
the `some_folder // 'sub_folder' // 'leaf'`
(double-slash) syntax.
- Fix a bug in traversal of public and archive folders. These folder hierarchies are now fully supported.
- Fix a bug where the timezone of a calendar item changed when the item was fetched and then saved.
- Kerberos support is now optional and Kerberos dependencies are not installed by default. Install
as `pip install exchangelib[kerberos]` to get Kerberos support.

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- Improve back off handling when receiving `ErrorServerBusy` error messages from the server
- Fixed bug where `Account.root` and its children would point to the root folder of the connecting account instead of
the target account when connecting to other accounts.

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- Add experimental Kerberos support. This adds the `pykerberos` package, which needs the following system packages to be
installed on Ubuntu/Debian systems: `apt-get install build-essential libssl-dev libffi-dev python-dev libkrb5-dev`.