Add beautifulsoup4 (w/ soupsieve)

Example:

If you do:

```js
languagePluginLoader.then(() => {
  // pyodide is now ready to use...
  pyodide.loadPackage('beautifulsoup4').then(() => {
    pyodide.runPython(`
text = """
<div>
  <!-- These are animals -->
  <p class="a">Cat</p>
  <p class="b">Dog</p>
  <p class="c">Mouse</p>
</div>
"""

from bs4 import BeautifulSoup
import soupsieve
soup = BeautifulSoup(text, 'html.parser')

print(soup.select('p:is(.a, .c)'))
    `);
  });
});
```

then the output is:

```
[<p class="a">Cat</p>, <p class="c">Mouse</p>]
```

packages/beautifulsoup4/meta.yaml

@@ -0,0 +1,12 @@
+package:
+  name: beautifulsoup4
+  version: 4.7.1
+source:
+  sha256: 945065979fb8529dd2f37dbb58f00b661bdbcbebf954f93b32fdf5263ef35348
+  url: https://files.pythonhosted.org/packages/80/f2/f6aca7f1b209bb9a7ef069d68813b091c8c3620642b568dac4eb0e507748/beautifulsoup4-4.7.1.tar.gz
+requirements:
+  run:
+  - soupsieve
+test:
+  imports:
+  - bs4

packages/soupsieve/meta.yaml

@@ -0,0 +1,9 @@
+package:
+  name: soupsieve
+  version: '1.8'
+source:
+  sha256: eaed742b48b1f3e2d45ba6f79401b2ed5dc33b2123dfe216adb90d4bfa0ade26
+  url: https://files.pythonhosted.org/packages/0c/52/e9088bb9b96e2d39fc3b33fcda5b4fde9d71473536ac660a1ca9a0958a2f/soupsieve-1.8.tar.gz
+test:
+  imports:
+  - soupsieve