Delete Python 2 leftover

Detected by pyugrade.

Author: DimitriPapadopoulos

numcodecs/tests/common.py

@@ -276,7 +276,7 @@ def check_backwards_compatibility(codec_id, arrays, codecs, precision=None, pref
                 with open(codec_fn, mode='w') as cf:
                     _json.dump(codec.get_config(), cf, sort_keys=True, indent=4)
             # load config and compare with expectation
-            with open(codec_fn, mode='r') as cf:
+            with open(codec_fn) as cf:
                 config = _json.load(cf)
                 assert codec == get_codec(config)
 

numcodecs/vlen.pyx

@@ -98,7 +98,7 @@ class VLenUTF8(Codec):
         for i in range(n_items):
             u = input_values[i]
             if u is None or u == 0:  # treat these as missing value, normalize
-                u = u''
+                u = ''
             elif not PyUnicode_Check(u):
                 raise TypeError('expected unicode string, found %r' % u)
             b = PyUnicode_AsUTF8String(u)