Support more url schemes for getting standalone manifest

urllib.requests.urlopen also supports file, so call it unless the
scheme is 'gs'.  This adds http, https, and ftp support.

Change-Id: I3f215c3ebd8e6dee29ba14c7e79ed99d37287109
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/322095
Reviewed-by: Michael Kelly <mkelly@arista.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Matt Story <mstory@arista.com>
diff --git a/fetch.py b/fetch.py
index d79f947..c954a9c 100644
--- a/fetch.py
+++ b/fetch.py
@@ -17,6 +17,8 @@
 import subprocess
 import sys
 from urllib.parse import urlparse
+from urllib.request import urlopen
+
 
 def fetch_file(url, verbose=False):
   """Fetch a file from the specified source using the appropriate protocol.
@@ -39,7 +41,5 @@
       print('fatal: error running "gsutil": %s' % e.stderr,
             file=sys.stderr)
     sys.exit(1)
-  if scheme == 'file':
-    with open(url[len('file://'):], 'rb') as f:
-      return f.read()
-  raise ValueError('unsupported url %s' % url)
+  with urlopen(url) as f:
+    return f.read()