BUG: Patch for skipping seek() when loading Excel files from url

Closes pandas-dev#20434.

Back in pandas-dev#19779 a call of a seek() method was added. This call fails
on HTTPResponse instances with an UnsupportedOperation exception,
so for this case a try..except wrapper was added here.

pandas/io/excel.py

@@ -388,8 +388,13 @@ def __init__(self, io, **kwds):
         elif not isinstance(io, xlrd.Book) and hasattr(io, "read"):
             # N.B. xlrd.Book has a read attribute too
             if hasattr(io, 'seek'):
-                # GH 19779
-                io.seek(0)
+                try:
+                    # GH 19779
+                    io.seek(0)
+                except (OSError, ValueError):
+                    # HTTPResponse does not support seek()
+                    # GH 20434
+                    pass
 
             data = io.read()
             self.book = xlrd.open_workbook(file_contents=data)