Pytgbot

Added `download_url` parameter to the constructor of the `Bot`s, so one can set the download url for non-official API servers.
Those might be different for a selfhosted server.
If that is not given but only a `base_url` is, we'll try to guess it but show a warning.
That is used in `get_download_url(…)`.

Fixed syntax error in `get_download_url(…)`.
Also cut a stable release (too early again).

Fixed Sync bot not resolving username and id correctly.

Fixed importing the wrong bot, the async one, per default.

This is a re-release of `5.0.0-beta.1`, as PyPi did like that short version format better.
Other than the version number it is exactly the same as `5.0.0-beta.1`.

Core:
- The already for a while now deprecated `pytgbot.api_types.sendable.InputFile`, `pytgbot.api_types.sendable.InputFileFromURL` and `pytgbot.api_types.sendable.InputFileFromDisk` can no longer be found in `pytgbot.api_types.sendable.\*`
They are now only available at `pytgbot.api_types.sendable.files.\*`, resulting in
- `pytgbot.api_types.sendable.files.InputFile`,
- `pytgbot.api_types.sendable.files.InputFileFromURL` and
- `pytgbot.api_types.sendable.files.InputFileFromDisk`.
- `.as_array()` now has a `prefer_original=False` boolean, if it should return the data this was constructed with if available. Otherwise, it will be constructed normally from the data of the object, as it was before.

Licence:
- Switched from GPL to LGPL.