Added the ability to use other instances of the mystbin application.

[ShadowFox88]

Description

This PR adds the ability to use self hosted instances of mystbin. Previously, the code relied on using the instance hosted by the PythonistaGuild at https://mystb.in, and had no way to change this to use any other instances. It keeps backwards compatibility in that it will always default to using https://mystb.in, but can be changed at will. Apologises in advance for any issues with documentation, I have tried to implement it as best as I could.

The code I used to test my changes is below. It is not comprehensive as I do not feel that the scope of what I am adding would have affected any other features, however it should be enough to verify that the code works.

import asyncio

import mystbin

file = mystbin.File(filename="File1.txt", content="Hello there!")
file2 = mystbin.File(filename="test.py", content="print('hello!')")


async def test() -> None:
    client = mystbin.Client()
    paste = await client.create_paste(files=[file, file2])
    print(paste)

async def test_with_custom_url() -> None:
    client = mystbin.Client(api_base="<mystbin instance here: in the form https://example.com>") # this could end with / or without and it works.
    paste = await client.create_paste(files=[file, file2])
    print(paste)

asyncio.run(test())
asyncio.run(test_with_custom_url())

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
  • I have updated the changelog with a quick recap of my changes.
• 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, ...)
• I have read and agree to the Developer Certificate of Origin for this contribution