Make TextChannel._get_channel async

[Qxe5]

Summary

TextChannel._get_channel() is called with await but isn't defined as async

For example:

pycord/discord/abc.py

Line 1428 in 4d80dd5

channel = await self._get_channel()

This leads to exceptions such as TypeError: object TextChannel can't be used in 'await' expression

This change is consistent with all other classes in the module such as VoiceChannel which do use async in the definition of _get_channel()

pycord/discord/channel.py

Line 1160 in 4d80dd5

async def _get_channel(self):

pycord/discord/abc.py

Line 1247 in 4d80dd5

async def _get_channel(self) -> MessageableChannel:

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)