Read data from the stream until ``separator`` is found.
On success, the data and separator will be removed from the
internal buffer (consumed). Returned data will include the
separator at the end.
Configured stream limit is used to check result. Limit sets the
maximal length of data that can be returned, not counting the
separator.
If an EOF occurs and the complete separator is still not found,
an IncompleteReadError exception will be raised, and the internal
buffer will be reset. The IncompleteReadError.partial attribute
may contain the separator partially.
If the data cannot be read because of over limit, a
LimitOverrunError exception will be raised, and the data
will be left in the internal buffer, so it can be read again.
The ``separator`` may also be a tuple of separators. In this
case the return value will be the shortest possible that has any
separator as the suffix. For the purposes of LimitOverrunError,
the shortest possible separator is considered to be the one that
matched.