S3OpenRead ignores the mode of 'r' opens always in 'rb'

[vinayan3]

The S3OpenRead always returns data as bytes in Python 3 which breaks things expecting str. The S3OpenRead needs to respect 'r' instead of returning everything as bytes.

[appierys]

It appears that now smart_open supports 'r' mode in 1.5.4?
However, I found that the speed of reading files drops significantly after smart_open 1.5.4.

For smart_open 1.5.3, the speed is:
2868923it [01:05, 43987.90it/s]

while smart_open 1.5.4 is:
41588it [01:35, 434.05it/s]
for the same S3 file.

[menshikh-iv]

Already fixed (doesn't reproduce with current master fbc82cc