Fix phemex symbol parsing

[GuillaumeDSM]

Phemex now describes its perpetual instruments as perpetualv2 which is crashing the feed constructor and preventing phemex feeds from starting.

• [ x ] - Tested
• [ x ] - Changelog updated
• [ ] - Tests run and pass
• [ ] - Flake8 run and all errors/warnings resolved
• [ ] - Contributors file updated (optional)