utils/getdeveloperlib.py: fix check_output() return value decoding

In Python 3.x, check_output() returns a "bytes" array, and not a
string. Its result needs to be decoded to be turned into a
string. Without this fix, "get-developers -c" bails out with:

Traceback (most recent call last):
  File "/home/thomas/projets/buildroot/./utils/get-developers", line 105, in <module>
    __main__()
  File "/home/thomas/projets/buildroot/./utils/get-developers", line 53, in __main__
    files = getdeveloperlib.check_developers(devs)
  File "/home/thomas/projets/buildroot/utils/getdeveloperlib.py", line 280, in check_developers
    files = subprocess.check_output(cmd).strip().split("\n")
TypeError: a bytes-like object is required, not 'str'

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

utils/getdeveloperlib.py

@@ -277,7 +277,7 @@ def check_developers(developers, basepath=None):
     if basepath is None:
         basepath = os.getcwd()
     cmd = ["git", "--git-dir", os.path.join(basepath, ".git"), "ls-files"]
-    files = subprocess.check_output(cmd).strip().split("\n")
+    files = subprocess.check_output(cmd).decode(sys.stdout.encoding).strip().split("\n")
     unhandled_files = []
     for f in files:
         handled = False