Annotating netsgiro 9/9: Make package PEP 561-compatible by adding py.typed

[sondrelg]

Relates to #14

https://peps.python.org/pep-0561/

https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages

Adding a py.typed file tells type checkers (e.g., mypy) to use the new type hints.

[codecov]

Codecov Report

Merging #68 (490fe9e) into master (2637c29) will decrease coverage by 0.31%.
The diff coverage is 96.07%.

❗ Current head 490fe9e differs from pull request most recent head 2d252dd. Consider uploading reports for the commit 2d252dd to get more accurate results

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   95.76%   95.45%   -0.32%     
==========================================
  Files           7        7              
  Lines         567      572       +5     
  Branches       94       94              
==========================================
+ Hits          543      546       +3     
- Misses         13       15       +2     
  Partials       11       11              

Impacted Files	Coverage Δ
netsgiro/records.py	93.87% <94.52%> (ø)
netsgiro/objects.py	96.16% <96.92%> (+0.01%)	⬆️
netsgiro/converters.py	91.48% <100.00%> (-3.86%)	⬇️

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[sourcery-ai]

Sourcery Code Quality Report

Merging this PR leaves code quality unchanged.

Quality metrics	Before	After	Change
Complexity	1.53 ⭐	1.53 ⭐	0.00
Method Length	18.40 ⭐	18.40 ⭐	0.00
Working memory	3.60 ⭐	3.60 ⭐	0.00
Quality	94.25% ⭐	94.25% ⭐	0.00%

Other metrics	Before	After	Change
Lines	88	88	0

Changed files	Quality Before	Quality After	Quality Change
netsgiro/converters.py	94.25% ⭐	94.25% ⭐	0.00%

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

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