Upgrade urllib3 and fix test

urllib3 v2.0.0 started enforcing that the amount of data it retrieves
from the server matches the `Content-Length` header by default. See
urllib3/urllib3#2514 .

This caused some tests to fail, exposing an issue where the mock HTTP
was setting invalid Content-Length and Content-Range headers.

setup.py

@@ -34,7 +34,7 @@
     ],
     install_requires = [
         "discord.py>=2.0.0,<3.0.0",
-        "urllib3>=1.26.12,<2.0.0",
+        "urllib3>=1.26.12,<3.0.0",
         "iterable-io>=1.0.0,<2.0.0",
     ],
     packages=["slack_to_discord"],

tests/test_http_stream.py

@@ -19,6 +19,7 @@
 def gen_bytes(s, e):
     return b"".join(bytes([x]) for x in range(s, e))
 
+
 class HTTPRangeRequestHandler(http.server.SimpleHTTPRequestHandler):
     def do_GET(self):
         range_ = self.headers.get("Range")
@@ -37,12 +38,12 @@ def do_GET(self):
                 return
             else:
                 self.send_response(HTTPStatus.PARTIAL_CONTENT)
-                self.send_header("Content-Range", f"bytes {start}-{end-1}/{end}")
+                self.send_header("Content-Range", f"bytes {start}-{end-1}/{RESP_SIZE}")
         else:
             self.send_response(HTTPStatus.OK)
 
         self.send_header("Accept-Ranges", "bytes")
-        self.send_header("Content-Length", RESP_SIZE)
+        self.send_header("Content-Length", end - start)
         self.end_headers()
         self.wfile.write(gen_bytes(start, end))