Please add dependency on Python 3.9 in README

[karen-pal]

• gdal2tiles version: Version: 0.1.9
• GDAL version:
• Python version: >=3.9
• Operating System: Linux

Description

New users should know that this only works on python 3.9 ATM

What I Did

I tried installing this library in python 3.11 - an arror appears saying :

    132 | Py_DEPRECATED(3.9) PyAPI_FUNC(void) PyEval_InitThreads(void);
            |                                     ^~~~~~~~~~~~~~~~~~
      error: command '/usr/bin/gcc' failed with exit code 1
      [end of output]

the solution is to downgrade python ATM. Please update the README.