UP015 RedundantOpenModes false positive with gzip.open

[andersk]

Ruff’s fix for UP015 (#843) incorrectly changes the behavior of gzip.open(…, "rt"). This mode should not be flagged as redundant because the default for gzip.open is "rb".

$ cat test.py
import gzip
with gzip.open("test.gz", "rt") as f:
    print(f.read())

$ echo 'Hello, world!' | gzip > test.gz

$ python test.py
Hello, world!

$ ruff --select=UP test.py
Found 1 error(s).
test.py:2:6: UP015 Unnecessary open mode parameters
1 potentially fixable with the --fix option.

$ ruff --select=UP --fix test.py
Found 0 error(s) (1 fixed).

$ cat test.py
import gzip
with gzip.open("test.gz") as f:
    print(f.read())

$ python test.py
b'Hello, world!\n'

Cc @andribergs