Enable Python 3.10's EncodingWarning

[AA-Turner]

This warns on the default encoding being used in open or io.open, which can lead to cross-platform bugs (for example on Windows where the default codepage is cp1252), or with the upcoming change to the default Python encoding (PEP 686, Make UTF-8 mode default https://peps.python.org/pep-0686/)

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I think this meets the bar for triviality so I haven't included a changelog entry. PR created per @nicoddemus's request in #10326, though this is not sufficient to close that issue.

A

[bluetech]

Needs a rebase now that #10328 is merged, although I highly doubt that was the only case missing an encoding -- probably just the first to error. I suspect that are many more places that will need to be fixed.

[inwaves]

I'm currently going through the codebase and updating calls to open without an encoding to include an encoding of UTF-8. There are around 500 in total, luckily most are trivial.

Some tests are failing because they use iniconfig, which does not specify an encoding when opening an .ini file. I've opened a matching issue on the repo.

[nicoddemus]

Thanks for tackling this @inwaves, we appreciate it.

Some tests are failing because they use iniconfig, which does not specify an encoding when opening an .ini file. I've opened a matching issue on the repo.

For ref: pytest-dev/iniconfig#48

[bluetech]

iniconfig had its unicode maniac release :) so let's try again

[bluetech]

Looking at the result I think it's undesirable to fix all of the EncodingWarnings in the test suite, even just the adding of 'utf-8' to all write_text calls will uglify the code considerably, and they are 99% (maybe even 100%) ascii anyway. I think it's better to just wait until it becomes the default in a few years. So my recommendation is to close this PR - if anyone disagrees, let me know.

I did find two places to fix in pytest itself (not its test suite), which will affect users who want to be clean. Will send a PR.

[bluetech]

Closing per the above, thanks for bringing this up @AA-Turner.