Port Datasette from Sanic to ASGI + Uvicorn (#518)

Datasette now uses ASGI internally, and no longer depends on Sanic.

It now uses Uvicorn as the underlying HTTP server.

This was thirteen months in the making... for full details see the issue:

https://github.com/simonw/datasette/issues/272

And for a full sequence of commits plus commentary, see the pull request:

https://github.com/simonw/datasette/pull/518

tests/test_html.py

@@ -8,6 +8,7 @@ from .fixtures import (  # noqa
     METADATA,
 )
 import json
+import pathlib
 import pytest
 import re
 import urllib.parse
@@ -16,6 +17,7 @@ import urllib.parse
 def test_homepage(app_client_two_attached_databases):
     response = app_client_two_attached_databases.get("/")
     assert response.status == 200
+    assert "text/html; charset=utf-8" == response.headers["content-type"]
     soup = Soup(response.body, "html.parser")
     assert "Datasette Fixtures" == soup.find("h1").text
     assert (
@@ -44,6 +46,29 @@ def test_homepage(app_client_two_attached_databases):
     ] == table_links
 
 
+def test_http_head(app_client):
+    response = app_client.get("/", method="HEAD")
+    assert response.status == 200
+
+
+def test_static(app_client):
+    response = app_client.get("/-/static/app2.css")
+    assert response.status == 404
+    response = app_client.get("/-/static/app.css")
+    assert response.status == 200
+    assert "text/css" == response.headers["content-type"]
+
+
+def test_static_mounts():
+    for client in make_app_client(
+        static_mounts=[("custom-static", str(pathlib.Path(__file__).parent))]
+    ):
+        response = client.get("/custom-static/test_html.py")
+        assert response.status == 200
+        response = client.get("/custom-static/not_exists.py")
+        assert response.status == 404
+
+
 def test_memory_database_page():
     for client in make_app_client(memory=True):
         response = client.get("/:memory:")