Fix deprecated np.unicode type (#2035)

Until numpy version 1.20.0, numpy.unicode was an alias for str in python3. In 1.20.0, it's fully deprecated and is an int. This is bad and breaks things. This commit drops the np.unicode alias and just uses str, as is advised here:
https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations

server/common/utils/type_conversion_utils.py

@@ -44,7 +44,7 @@ def get_dtype_from_dtype(dtype, array_values=None):
     if dtype_name == "bool":
         return np.uint8
     if dtype_name == "object" and dtype_kind == "O":
-        return np.unicode
+        return str
     if dtype_name == "category":
         return get_dtype_from_dtype(dtype.categories.dtype, array_values)