magic-proxy: fix TypeError when trying to call get_uri() LP:#1944906

Currently the uri that is passed into urllib.parse.urlparse() is not
prefixed with "http(s)://" which leads urlparse() to return a wrong
scheme/netloc/path. Currently it looks like:

ParseResult(scheme='', netloc='',
  path='de.archive.ubuntu.com/ubuntu/dists/impish-backports/InRelease'
 , params='', query='', fragment='')

That's wrong. The path should look like
'ubuntu/dists/impish-backports/InRelease'.
Prefixing the 'host' header with 'http://' in case it's not there does
fix the problem.

This fixes:

Traceback (most recent call last):
  File "/usr/lib/python3.9/socketserver.py", line 683, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python3.9/socketserver.py", line 360, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python3.9/socketserver.py", line 747, in __init__
    self.handle()
  File "/usr/lib/python3.9/http/server.py", line 427, in handle
    self.handle_one_request()
  File "/usr/lib/python3.9/http/server.py", line 415, in handle_one_request
    method()
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 787, in do_GET
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 838, in __get_request
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 84, in get_uri
TypeError: can only concatenate str (not "NoneType") to str