Fix: deprecated and now retired "universal newlines" mode

[tardate]

I just tried out csv2md on Python 3.11.10.

It fails with the error:

Traceback (most recent call last):
  File "/opt/homebrew/bin/csvtomd", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/csvtomd/csvtomd.py", line 149, in main
    with open(filename, 'rU') as f:
         ^^^^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'rU'

As I understand it, this is because the "universal newlines" mode ('rU') has been deprecated since Python 3.4 and completely removed in Python 3.11.

I haven't dug into the code yet to see if I can suggest a PR. First just wanted to ask if there is an appetite/intent to upgrade for newer python versions?