pyntcore: Add repr for entries and topics

[auscompgeek]

Open questions:

• should the repr quote the key? i.e. <IntegerTopic '/int'>
  • teams might include weird things in their keys, in particular keys containing > might make it difficult to read nested reprs
• should Topic et al's reprs include the ntcore module name?
  • technically it's possible to subclass the base Topic class (so it could be wrong), but not in a meaningful way

[virtuald]

LGTM, I don't think you need to quote the keys, they're just for debugging purposes anyways.

[auscompgeek]

they're just for debugging purposes anyways

Yeah, I just wanna optimise for readability here.