servo: Merge #12510 - Upgrade mozlog package from 3.0 to 3.3 (from chenpighead:update-mozlog); r=metajack

<!-- Please describe your changes on the following line: -->
Update mozlog to 3.3 so we can have correct reftest screenshots displayed with --log-html.
The original mozlog update bugs are listed as follows:
https://bugzilla.mozilla.org/show_bug.cgi?id=1287019
https://bugzilla.mozilla.org/show_bug.cgi?id=1287480

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12401 and #12403 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this is just a PyPI package version bumping

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Update mozlog to 3.3.

Source-Repo: https://github.com/servo/servo
Source-Revision: 73cfab0016fd761da24d6280154fda6561b417fe

servo/python/requirements.txt

@@ -2,7 +2,7 @@ blessings == 1.6
 mach == 0.6.0
 mozdebug == 0.1
 mozinfo == 0.8
-mozlog == 3.0
+mozlog == 3.3
 setuptools == 18.5
 toml == 0.9.1