scripts: Replace basestring -> str for python3

Python 3 doesn't have basestring type as all string
are unicode strings.

(From OE-Core rev: e8cfab060f4ff3c4c16387871354d407910e87aa)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/files/ext-sdk-prepare.py

@@ -8,7 +8,7 @@ import subprocess
 
 def exec_watch(cmd, **options):
     """Run program with stdout shown on sys.stdout"""
-    if isinstance(cmd, basestring) and not "shell" in options:
+    if isinstance(cmd, str) and not "shell" in options:
         options["shell"] = True
 
     process = subprocess.Popen(