nanovna-saver.py: fix execution from outside source directory

[silbe]

nanovna-saver.py can be called from outside the source directory. The module import path needs to be resolved relative to the source directory, not relative to the current working directory of the process.

Fixes: b011000 ("moved to pyscaffold directory structure")

This branch is based on the commit before the transition to PyQt6 as the latter is not available in Debian stable (Bullseye).

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

When executing nanovna-saver.py from outside the source directory but without installing it, importing main fails:

I have no name!@nanovna:~/nanovna-data$ cd ~/nanovna-data && python3 ~/nanovna-saver/nanovna-saver.py -d
Traceback (most recent call last):
  File "/home/sascha/nanovna-saver/nanovna-saver.py", line 29, in <module>
    from NanoVNASaver.__main__ import main
ModuleNotFoundError: No module named 'NanoVNASaver.__main__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/sascha/nanovna-saver/nanovna-saver.py", line 35, in <module>
    from NanoVNASaver.__main__ import main
ModuleNotFoundError: No module named 'NanoVNASaver.__main__'

Issue Number: N/A

What is the new behavior?

Running nanovna-saver.py from outside the source directory (but without installing it first) works again.

Does this introduce a breaking change?

• Yes
• No

Other information

[silbe]

Wow, that was quick. Thanks for review & merge!