Adapt to Python 3.9 API changes

[lorenzph]

While attempting to save a project on my machine using Python 3.9 I ran into the following traceback.

Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/cura/CuraApplication.py", line 1063, in event
    return super().event(event)
  File "/usr/lib/python3.9/site-packages/UM/Qt/QtApplication.py", line 464, in event
    event._function_event.call()
  File "/usr/lib/python3.9/site-packages/UM/Event.py", line 218, in call
    self._function(*self._args, **self._kwargs)
  File "/usr/lib/python3.9/site-packages/cura/Utils/Threading.py", line 34, in _handle_call
    ico.result = func(*args, **kwargs)
  File "/usr/lib/cura/plugins/3MFWriter/ThreeMFWorkspaceWriter.py", line 57, in write
    self._writeContainerToArchive(container, archive)
  File "/usr/lib/cura/plugins/3MFWriter/ThreeMFWorkspaceWriter.py", line 154, in _writeContainerToArchive
    serialized_data = container.serialize(ignored_metadata_keys = ignore_keys)
  File "/usr/lib/cura/plugins/XmlMaterialProfile/XmlMaterialProfile.py", line 154, in serialize
    builder.start("metadata") # type: ignore
TypeError: start expected 2 arguments, got 1

This was introduced by a fix in Python 3.9 which aligns the C implementation of TreeBuilder with the Python implementation.

[nallath]

It doesn't break anything for us, so I we might as well include it. Thanks for the contribution!

[deBugger-]

@nallath
Hi,
are you so kind to tell us which phyton version works. Better a link to a detail cura precondition documentation.

https://support.ultimaker.com/hc/en-us/articles/360011788040-What-are-the-system-requirements-for-Ultimaker-Cura-
On this page are the details missing.

Thank you

[nallath]

The details are missing there because the installer handles that. We don't use any pre-installed python versions if you download it.

If you want to run from source, Python 3.8.6 is what I use

[Ghostkeeper]

https://support.ultimaker.com/hc/en-us/articles/360011788040-What-are-the-system-requirements-for-Ultimaker-Cura-
On this page are the details missing.

The details are missing there because it is assuming you are using the official distribution of Cura, which comes with its own copy of CPython, rather than using your system's Python installation.