Fix error when calling str() or repr() on GCS SeekableBufferedInputBase

[robcowie]

Fix __repr__ and __str__ on gcs.SeekableBufferedInputBase by storing the bucket as _bucket instead of bucket.

This is now consistent with gcs.BufferedOutputBase and s3. _SeekableRawReader (which stores its S3 client as _object)

Motivation

We were seeing logging errors originating from smart_open:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1081, in emit msg = self.format(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 925, in format return fmt.format(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 664, in format record.message = record.getMessage()
  File "/usr/local/lib/python3.8/logging/__init__.py", line 369, in getMessage msg = msg % self.args
  File "/usr/local/lib/python3.8/gzip.py", line 220, in __repr__ s = repr(self.fileobj)
  File "/usr/local/lib/python3.8/site-packages/smart_open/gcs.py", line 379, in __repr__ self.__class__.__name__, self._bucket.name, self._blob.name, self._current_part_size, AttributeError: 'SeekableBufferedInputBase' object has no attribute '_bucket'

[mpenkov]

Merged. Thank you @robcowie for your contribution!