fix deprecation warning from tomli

[MarcoGorelli]

Description

Checklist - did you ...

• Add a CHANGELOG entry if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

Currently, running tests shows

  /Users/marcogorelli/black-dev/venv/lib/python3.8/site-packages/tomli/_parser.py:71: DeprecationWarning: Text file object support is deprecated in favor of binary file objects. Use `open("foo.toml", "rb")` to open the file in binary mode.
    warnings.warn(

From the tomli README:

The file must be opened in binary mode (with the "rb" flag). Binary mode will enforce decoding the file as UTF-8 with universal newlines disabled, both of which are required to correctly parse TOML. Support for text file objects is deprecated for removal in the next major release.

[hukkin]

Dupe of #2408 😺

[MarcoGorelli]

didn't notice that, thanks!