Add support for Python 3.13 and drop EOL 3.7-3.8

The make build failures are from twine check:

build: commands[1]> twine check 'dist/*.whl' 'dist/*.tar.gz'
Checking dist/pytablewriter-1.2.0-py3-none-any.whl: FAILED
ERROR    `long_description` has syntax errors in markup and would not be        
         rendered on PyPI.                                                      
         line 156: Warning: Cannot scale image!                                 
           Could not get size from                                              
         "https://cdn.jsdelivr.net/gh/thombashi/pytablewriter@master/docs/pages/
         examples/table_format/text/ss/markdown.png":                           
           Requires Python Imaging Library.                                     
           Reading external files disabled.                                     
Checking dist/pytablewriter-1.2.0.tar.gz: FAILED
ERROR    `long_description` has syntax errors in markup and would not be        
         rendered on PyPI.                                                      
         line 156: Warning: Cannot scale image!                                 
           Could not get size from                                              
         "https://cdn.jsdelivr.net/gh/thombashi/pytablewriter@master/docs/pages/
         examples/table_format/text/ss/markdown.png":                           
           Requires Python Imaging Library.                                     
           Reading external files disabled.

This is caused by https://github.com/pypa/readme_renderer/issues/304.

Two suggestions:

remove the :scale: parts from README.rst
programmatically remove them when reading the file in setup.py

thombashi / pytablewriter

Add support for Python 3.13 and drop EOL 3.7-3.8 #65