scmutil: fix requires-file isalnum() check on first byte

Slice instead of subscript to get a bytes from a bytes.

# skip-blame just a py3 slice-instead-of-subscript change

Differential Revision: https://phab.mercurial-scm.org/D2341

mercurial/scmutil.py

@@ -887,7 +887,7 b' def readrequires(opener, supported):'
     missings = []
     for r in requirements:
         if r not in supported:
-            if not r or not r[0].isalnum():
+            if not r or not r[0:1].isalnum():
                 raise error.RequirementError(_(".hg/requires file is corrupt"))
             missings.append(r)
     missings.sort()