remove unused depencencies (#897)

flask_restful/fields.py

@@ -1,6 +1,4 @@
-from datetime import datetime
 from calendar import timegm
-import pytz
 from decimal import Decimal as MyDecimal, ROUND_HALF_EVEN
 from email.utils import formatdate
 import six
@@ -9,8 +7,7 @@
 except ImportError:
     # python3
     from urllib.parse import urlparse, urlunparse
-
-from flask_restful import inputs, marshal
+from flask_restful import marshal
 from flask import url_for, request
 
 __all__ = ["String", "FormattedString", "Url", "DateTime", "Float",