python3.lang: support PEP-515, drop obsolete long-integer suffix

Python3 language improvements:

• remove obsolete (py2) long-integer suffixes
• support integers underscore grouping, as per PEP-515

References:

• https://docs.python.org/3/reference/lexical_analysis.html#integer-literals
• https://peps.python.org/pep-0515/