Consider specifying minimal Python3 version in requirements

[Aj0SK]

Consider updating the requirements in README.md as the input-generator command (running using Python 3.9.2) fails with:

andrejkorman@xxx:~/Documents/myoi$ input-generator idf.txt 
Traceback (most recent call last):
  File "/home/andrejkorman/.local/bin/input-generator", line 31, in <module>
    from input_tool.common.parser import Parser, ArgsGenerator
  File "/home/andrejkorman/.local/lib/python3.9/site-packages/input_tool/common/parser.py", line 8, in <module>
    class ArgsSample:
  File "/home/andrejkorman/.local/lib/python3.9/site-packages/input_tool/common/parser.py", line 16, in ArgsSample
    task: str | None
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

I suspect this is because this type of feature is rather new? It's easy to fix but not all first-time users may find this easy to debug.