UTF-8: Implement support in Python client library

[ywwg]

As in prometheus/client_golang#1369 and prometheus/client_java#916, the Python library needs to be updated to support UTF-8.

Tasks:

• Add flag to enable change in validation logic to check that metric and label names are UTF-8 valid instead of the old letters/numbers/underscores/colons set
• Update exposition format parsers for the new syntax
• Update PromQL parsers (if any) for the new syntax
• Update content negotiation logic

For background and references see prometheus/prometheus#13095

[ywwg]

I will take this