diff --git a/flask_mongoengine/operation_tracker.py b/flask_mongoengine/operation_tracker.py
index c52fb6b8..5f1c8f6b 100644
--- a/flask_mongoengine/operation_tracker.py
+++ b/flask_mongoengine/operation_tracker.py
@@ -43,9 +43,6 @@
 removes = []
 response_sizes = []
 
-if sys.version_info >= (3, 0):
-    unicode = str
-
 
 # Wrap helpers._unpack_response for getting response size
 @functools.wraps(_original_methods["_unpack_response"])
@@ -581,13 +578,6 @@ def _tidy_stacktrace():
             hidden = True
         if not text:
             text = ""
-        else:
-            if sys.version_info >= (3, 0):
-                text = "".join(text).strip()
-            else:
-                try:
-                    text = unicode("".join(text).strip())
-                except Exception:
-                    pass
+        text = "".join(text).strip()
         trace.append((path, line_no, func_name, text, hidden))
     return trace, internal
diff --git a/flask_mongoengine/sessions.py b/flask_mongoengine/sessions.py
index 0d14ff22..49b04051 100644
--- a/flask_mongoengine/sessions.py
+++ b/flask_mongoengine/sessions.py
@@ -1,5 +1,4 @@
 import datetime
-import sys
 import uuid
 
 from bson.tz_util import utc
@@ -8,9 +7,6 @@
 
 __all__ = ("MongoEngineSession", "MongoEngineSessionInterface")
 
-if sys.version_info >= (3, 0):
-    basestring = str
-
 
 class MongoEngineSession(CallbackDict, SessionMixin):
     def __init__(self, initial=None, sid=None):
@@ -33,8 +29,8 @@ def __init__(self, db, collection="session"):
         :param collection: The session collection name defaults to "session"
         """
 
-        if not isinstance(collection, basestring):
-            raise ValueError("collection argument should be string or unicode")
+        if not isinstance(collection, str):
+            raise ValueError("Collection argument should be string")
 
         class DBSession(db.Document):
             sid = db.StringField(primary_key=True)
@@ -82,6 +78,8 @@ def save_session(self, app, session, response):
         domain = self.get_cookie_domain(app)
         httponly = self.get_cookie_httponly(app)
 
+        # If the session is modified to be empty, remove the cookie.
+        # If the session is empty, return without setting the cookie.
         if not session:
             if session.modified:
                 response.delete_cookie(app.session_cookie_name, domain=domain)
diff --git a/flask_mongoengine/wtf/fields.py b/flask_mongoengine/wtf/fields.py
index b2c895c3..1743e801 100644
--- a/flask_mongoengine/wtf/fields.py
+++ b/flask_mongoengine/wtf/fields.py
@@ -2,7 +2,6 @@
 Useful form fields for use with the mongoengine.
 """
 import json
-import sys
 from gettext import gettext as _
 
 from mongoengine.queryset import DoesNotExist
@@ -15,9 +14,6 @@
     "QuerySetSelectField",
 )
 
-if sys.version_info >= (3, 0):
-    unicode = str
-
 
 class QuerySetSelectField(SelectFieldBase):
     """
@@ -171,10 +167,11 @@ def __init__(self, label="", validators=None, model=None, **kwargs):
 
 class JSONField(TextAreaField):
     def _value(self):
+        # TODO: Investigate why raw mentioned.
         if self.raw_data:
             return self.raw_data[0]
         else:
-            return self.data and unicode(json.dumps(self.data)) or ""
+            return self.data and json.dumps(self.data) or ""
 
     def process_formdata(self, value):
         if value:
diff --git a/flask_mongoengine/wtf/orm.py b/flask_mongoengine/wtf/orm.py
index 6cc9e936..a177c9c9 100644
--- a/flask_mongoengine/wtf/orm.py
+++ b/flask_mongoengine/wtf/orm.py
@@ -2,7 +2,6 @@
 Tools for generating forms based on mongoengine Document schemas.
 """
 import decimal
-import sys
 from collections import OrderedDict
 
 from bson import ObjectId
@@ -24,10 +23,6 @@
 )
 
 
-if sys.version_info >= (3, 0):
-    unicode = str
-
-
 def converts(*args):
     def _inner(func):
         func._converter_for = frozenset(args)
@@ -234,7 +229,7 @@ def coerce(self, field_type):
             "DecimalField": decimal.Decimal,
             "ObjectIdField": ObjectId,
         }
-        return coercions.get(field_type, unicode)
+        return coercions.get(field_type, str)
 
 
 def model_fields(model, only=None, exclude=None, field_args=None, converter=None):
diff --git a/setup.cfg b/setup.cfg
index c4731f72..0e46ca2d 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,7 +1,6 @@
 [tool:pytest]
 addopts = --cov=flask_mongoengine --cov-config=setup.cfg
 testpaths = tests
-env_override_existing_values = 1
 
 [flake8]
 ignore=E501,F403,F405,I201,W503,E203