tidy: fix open() encoding

[reidpr]

Split from #1860. Pylint: W1514 unspecified-encoding

This happens twice:

charliecloud/lib/filesystem.py

Line 892 in 5ceefa7

with open(self, "rt") as fp:

Probably right here.

charliecloud/lib/charliecloud.py

Line 683 in d679373

sys.stdout = open(os.devnull, 'w')

This one is a false positive: /dev/null discards all bytes written and offers no bytes on reading (just EOF), so encoding doesn’t matter. I think I might file a feature request on Pylint.