tools: ynl: add the Python requirements.txt file

It is a good practice to state explicitly which are the required Python
packages needed in a particular project to run it. The most commonly
used way is to store them in the `requirements.txt` file*.

*URL: https://pip.pypa.io/en/stable/reference/requirements-file-format/

Currently user needs to figure out himself that Python needs `PyYAML`
and `jsonschema` (and theirs requirements) packages to use the tool.
Add the `requirements.txt` for user convenience.

How to use it:
1) (optional) Create and activate empty virtual environment:
python3.X -m venv venv3X
source ./venv3X/bin/activate
2) Install all the required packages:
pip install -r requirements.txt
or
python -m pip install -r requirements.txt
3) Run the script!

The `requirements.txt` file was tested for:
* Python 3.6
* Python 3.8
* Python 3.10

Signed-off-by: Michal Michalik <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/net/ynl/requirements.txt

···
+jsonschema==4.*
+PyYAML==6.*