Convert tidy to a non-egg Python package

It seems that servo-tidy is only used by webrender in my GitHub
searches. WebRender could simply use `rustfmt` and the tidy on pypi
hasn't been updated since 2018. Converting tidy to a normal Python
package removes the maintenance burden of continually fixing the easy
install configuration.

Fixes #29094.
Fixes #29334.

python/tidy/tests/ban.rs

@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#![feature(plugin)]
+#![plugin(script_plugins)]
+
+extern crate js;
+
+use js::jsval::JSVal;
+use std::cell::Cell;
+use std::cell::UnsafeCell;
+
+struct Foo {
+    bar: Cell<JSVal>,
+    //~^ ERROR Banned type Cell<JSVal> detected. Use MutDom<JSVal> instead
+    foo: UnsafeCell<JSVal>
+    //~^ NOT AN ERROR
+}
+
+fn main() {}