Use UTF-8 for log file.

[methane]

What's the problem this feature will solve?

• If default text encoding is not UTF-8 (e.g. Windows users), pip might fail to log non-ASCII texts (e.g. non-ASCII paths on Windows).
• When using PYTHONWARNDEFAULTENCODING=1 (Python 3.10), dictConfg() emit EncodingWarning:

/home/inada-n/pyenv/versions/3.10.0b2/lib/python3.10/logging/config.py:748: EncodingWarning: 'encoding' argument not specified
  result = factory(**kwargs)

Describe the solution you'd like

diff --git a/src/pip/_internal/utils/logging.py b/src/pip/_internal/utils/logging.py
index 0569b9248..39a18fd6c 100644
--- a/src/pip/_internal/utils/logging.py
+++ b/src/pip/_internal/utils/logging.py
@@ -375,6 +375,7 @@ def setup_logging(verbosity, no_color, user_log_file):
                     "level": "DEBUG",
                     "class": handler_classes["file"],
                     "filename": additional_log_file,
+                    "encoding": "utf-8",
                     "delay": True,
                     "formatter": "indent_with_timestamp",

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