Confuse

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- Remove a `<4` Python version requirement bound.

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- Drop support for versions of Python below 3.6.

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- Add support for reading configuration values from environment variables
(see `EnvSource`).
- Resolve a possible race condition when creating configuration directories.

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- A new `Configuration.reload` method makes it convenient to reload and
re-parse all YAML files from the file system.

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- A new `MappingValues` template behaves like `Sequence` but for mappings with
arbitrary keys.
- A new `Optional` template allows other templates to be null.
- `Filename` templates now have an option to resolve relative to a specific
directory. Also, configuration sources now have a corresponding global
option to resolve relative to the base configuration directory instead of
the location of the specific configuration file.
- There is a better error message for `Sequence` templates when the data from
the configuration is not a sequence.

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- `pathlib.PurePath` objects can now be converted to `Path` templates.
- `AttrDict` now properly supports (over)writing attributes via dot notation.