EI-133: Improve setup

[ksu00]

This patch improves the setup by:

• Splitting up dev dependencies into dev and test dependencies. Currently, the test dependencies consists of only pytest, but more are to come in the future. 😃
• Making dependency groups optional.
• Updating the setup instructions in the README.

Additional changes:

• Use same version range for python as other already set up repos of the nightskycam project.
• Update pytest version range in order to enable version solving by poetry when setting up this repo as a submodule of nightskycam.
• Update .gitignore.
• Add __init__.py to tests directory.

Closes EI-133

[vincentberenz]

all good

[vincentberenz]

@ksu00 Hello, your updates broke the continuous integration on github. You will need to fix before being able to merge. Continuous integration setup is in .github/workflows