csv_metadata_quality: Move scoped imports to global

According to PEP8 we should avoid scoped imports unless you have a
good reason. Here there are two cases where we do (issn and isbn),
but I will move the others to the global scope.

csv_metadata_quality/fix.py

@@ -1,7 +1,10 @@
 import re
+from unicodedata import normalize
 
 import pandas as pd
 
+from csv_metadata_quality.util import is_nfc
+
 
 def whitespace(field, field_name):
     """Fix whitespace issues.
@@ -212,9 +215,6 @@ def normalize_unicode(field, field_name):
     Return normalized string.
     """
 
-    from csv_metadata_quality.util import is_nfc
-    from unicodedata import normalize
-
     # Skip fields with missing values
     if pd.isna(field):
         return