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_utils.py

@@ -3,11 +3,11 @@ Tests for various datasette helper functions.
 """
 
 from datasette import utils
+from datasette.utils.asgi import Request
 from datasette.filters import Filters
 import json
 import os
 import pytest
-from sanic.request import Request
 import sqlite3
 import tempfile
 from unittest.mock import patch
@@ -53,7 +53,7 @@ def test_urlsafe_components(path, expected):
     ],
 )
 def test_path_with_added_args(path, added_args, expected):
-    request = Request(path.encode("utf8"), {}, "1.1", "GET", None)
+    request = Request.fake(path)
     actual = utils.path_with_added_args(request, added_args)
     assert expected == actual
 
@@ -67,11 +67,11 @@ def test_path_with_added_args(path, added_args, expected):
     ],
 )
 def test_path_with_removed_args(path, args, expected):
-    request = Request(path.encode("utf8"), {}, "1.1", "GET", None)
+    request = Request.fake(path)
     actual = utils.path_with_removed_args(request, args)
     assert expected == actual
     # Run the test again but this time use the path= argument
-    request = Request("/".encode("utf8"), {}, "1.1", "GET", None)
+    request = Request.fake("/")
     actual = utils.path_with_removed_args(request, args, path=path)
     assert expected == actual
 
@@ -84,7 +84,7 @@ def test_path_with_removed_args(path, args, expected):
     ],
 )
 def test_path_with_replaced_args(path, args, expected):
-    request = Request(path.encode("utf8"), {}, "1.1", "GET", None)
+    request = Request.fake(path)
     actual = utils.path_with_replaced_args(request, args)
     assert expected == actual
 
@@ -363,7 +363,7 @@ def test_table_columns():
     ],
 )
 def test_path_with_format(path, format, extra_qs, expected):
-    request = Request(path.encode("utf8"), {}, "1.1", "GET", None)
+    request = Request.fake(path)
     actual = utils.path_with_format(request, format, extra_qs)
     assert expected == actual