Auto merge of #23400 - staktrace:patch-1, r=jdm

Include missing dependency requirements

Running `servo-tidy` in a virtualenv where all you've done is `pip install servo-tidy==0.3.0` doesn't actually work because of missing dependencies. PyYAML and voluptuous are required as well. I don't know anything about python package management but declaring these missing dependencies seems like an appropriate thing to do. Patch itself is untested because I don't know how, although with these dependencies installed in my virtualenv running `servo-tidy` works.

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python/tidy/setup.py

@@ -17,6 +17,8 @@ install_requires = [
     "flake8==2.4.1",
     "toml==0.9.2",
     "colorama==0.3.7",
+    "voluptuous==0.11.5",
+    "PyYAML==5.1",
 ]
 
 here = os.path.dirname(os.path.abspath(__file__))