Fixed double slash in binance futures open interest url and missing 'next_funding_time' in Bybit.

[agijsberts]

Description of code - what bug does this fix / what feature does this add?

This small PR fixes https://github.com/bmoscon/cryptofeed/issues/869. Credits to @TnzGit for suggesting the solution. It also addresses KeyErrors I regularly encounter when 'next_funding_time' is not present in a funding update on Bybit. Finally, it cleans some tiny flake8 errors/warnings regarding whitespace.

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

[agijsberts]

I repeatedly wrote backslash, but it was of course a (forward) slash...