'1abc:def' is being considered a valid relative URI

[Silvanoc]

Describe the bug
The string "1abc:def" is being considered a valid relative URI.
But, if I'm reading the specification right, it isn't a valid relative URI.

Expected result
uritools.isrelpath("1abc:def") returns False

Actual result
uritools.isrelpath("1abc:def") returns True

Reproduction steps

import uritools
print(uritools.isrelpath("1abc:def"))

[tkem]

Please see #74.