Replace a block of text by an `attention` admonition to highlight it
properly. Furthermore, add a missing comma.

...so that it can be used by the HTTP API.

The respective exception marks an error in the program, which should
correctly be an `AssertionError`.

`GlobalReference` has been renamed to `ExternalReference` in V3, but
this enum member has been missed in the rename.

lxml supports paths already, no modification is necessary there.
However, the `lxml.etree.ElementTree.write()` function requires
`BinaryIO`, i.e. files opened with the 'b' mode. While it would be
possible to access the underlying binary buffer of files opened in text
mode via `open()`, this isn't possible for `io.StringIO()`, as it
doesn't have the `buffer` property. Thus, even if we could support files
opened via `open()` in text mode, we couldn't annotate the XML
serialization functions with `TextIO`, as `io.StringIO()` remains
unsupported. Because of that, I decided to not support `TextIO` for the
XML serialization.

The builtin JSON module only supports file handles, with the
`json.dump()` method only supporting `TextIO` and `json.load()`
supporting `TextIO` and `BinaryIO`. Thus, the JSON adapter is modified
to `open()` given paths, while the JSON serialization is additionally
modified to wrap `BinaryIO` with `io.TextIOWrapper`.

Fix #42

... by using `StringIO` instead of `BytesIO`.

Resolution of id_short paths is added via
`UniqueIdShortNamespace.get_referable()`, such that it can be used on
every object, that spans such a namespace. `ModelReference.resolve()`
is simplified to make use of this new functionality. Furthermore,
tests for this are added.

Show the object, where the resolution failed, in the error messages.

A duplicate test is replaced by this test.