Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

* test_sync_write_decode_surrogate: utf-8 decode

When SyncWrite decodes bytes as utf-8, it should replace unknown
sequences with the unicode surrogate codepoint instead of crashing the
program.

Test case for #2969

* SyncWrite: decode with errors='surrogateescape'

Avoid bubbling UnicodeDecodeError up from stream handling internals.

Tox has no way of knowing that the bytestream emitted by a command will
be valid utf-8, even if utf-8 is ostensibly the "correct" encoding for
the stream. It's always possible for an arbitrary command to return
non-utf-8 bytes, and this situation should not break tox.

Fix #2969

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