File-object separator colon parsing PR #976 must be fixed before the next release

[jpivarski]

In #976, the parsing that determines where a colon is between a file name and an object path was changed incorrectly: a basic (and important) use-case fails:

>>> import uproot
>>> tree = uproot.open("00376186-543E-E311-8D30-002618943857.root:Events")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jpivarski/irishep/uproot5/src/uproot/reading.py", line 143, in open
    file = ReadOnlyFile(
  File "/home/jpivarski/irishep/uproot5/src/uproot/reading.py", line 584, in __init__
    Source, file_path = uproot._util.file_path_to_source_class(
  File "/home/jpivarski/irishep/uproot5/src/uproot/_util.py", line 508, in file_path_to_source_class
    raise ValueError(f"URI scheme not recognized: {file_path}")
ValueError: URI scheme not recognized: 00376186-543E-E311-8D30-002618943857.root:Events

but

>>> tree = uproot.open("/tmp/00376186-543E-E311-8D30-002618943857.root:Events")

passes. The failing case is a relative local path; the successful case is an absolute local path.

I verified that 6b1952e (just before #976) passes but fdcbb8e (just after #976) fails.