Rollup merge of #57358 - euclio:docck-unicode, r=QuietMisdreavus

use utf-8 throughout htmldocck

This commit improves compatibility with Python 3, which already uses
Unicode throughout.

It also fixes a subtle incompatibility stemming from the use of
`entitydefs`, which contains replacement text _encoded in latin-1_ for
HTML entities. When using Python 3, this would cause `0xa0` to be
incorrectly added to the element tree.

This meant that there was a rustdoc test that would pass under Python 2
but fail under Python 3, due to an incorrect regex match against the
non-breaking space character. This commit triggers that failure in both
versions, and also fixes it.

src/test/rustdoc/issue-32374.rs

@@ -10,7 +10,7 @@
 //      'Deprecated since 1.0.0: text'
 // @has - '<code>test</code>&nbsp;<a href="http://issue_url/32374">#32374</a>'
 // @matches issue_32374/struct.T.html '//*[@class="stab unstable"]' \
-//      '🔬 This is a nightly-only experimental API. \(test #32374\)$'
+//      '🔬 This is a nightly-only experimental API. \(test\s#32374\)$'
 /// Docs
 #[rustc_deprecated(since = "1.0.0", reason = "text")]
 #[unstable(feature = "test", issue = "32374")]