[3.0] Use `open` instead of `codecs.open` to fix line breaks

codecs.open() splits lines at unexpected characters like 0x1d.

Switch open_file_*() to use open() which does not have this problem. (Note: 3.1 and master already use open().)

This fixes the test failure introduced in the 3.0 branch in commit ea681c46 which contains an example log with a 0x1d character.