diff --git a/html5lib/_inputstream.py b/html5lib/_inputstream.py
index 0207dd21..a93b5a4e 100644
--- a/html5lib/_inputstream.py
+++ b/html5lib/_inputstream.py
@@ -324,7 +324,7 @@ def charsUntil(self, characters, opposite=False):
         except KeyError:
             if __debug__:
                 for c in characters:
-                    assert(ord(c) < 128)
+                    assert ord(c) < 128
             regex = "".join(["\\x%02x" % ord(c) for c in characters])
             if not opposite:
                 regex = "^%s" % regex
diff --git a/html5lib/serializer.py b/html5lib/serializer.py
index c66df683..a171ac1c 100644
--- a/html5lib/serializer.py
+++ b/html5lib/serializer.py
@@ -222,14 +222,14 @@ def __init__(self, **kwargs):
         self.strict = False
 
     def encode(self, string):
-        assert(isinstance(string, text_type))
+        assert isinstance(string, text_type)
         if self.encoding:
             return string.encode(self.encoding, "htmlentityreplace")
         else:
             return string
 
     def encodeStrict(self, string):
-        assert(isinstance(string, text_type))
+        assert isinstance(string, text_type)
         if self.encoding:
             return string.encode(self.encoding, "strict")
         else:
diff --git a/html5lib/tests/test_serializer.py b/html5lib/tests/test_serializer.py
index bce62459..a2be0be5 100644
--- a/html5lib/tests/test_serializer.py
+++ b/html5lib/tests/test_serializer.py
@@ -74,7 +74,7 @@ def _convertAttrib(self, attribs):
         attrs = {}
         for attrib in attribs:
             name = (attrib["namespace"], attrib["name"])
-            assert(name not in attrs)
+            assert name not in attrs
             attrs[name] = attrib["value"]
         return attrs
 
diff --git a/html5lib/treebuilders/etree.py b/html5lib/treebuilders/etree.py
index 086bed4e..0b745081 100644
--- a/html5lib/treebuilders/etree.py
+++ b/html5lib/treebuilders/etree.py
@@ -108,7 +108,7 @@ def removeChild(self, node):
             node.parent = None
 
         def insertText(self, data, insertBefore=None):
-            if not(len(self._element)):
+            if not len(self._element):
                 if not self._element.text:
                     self._element.text = ""
                 self._element.text += data
@@ -201,7 +201,7 @@ def testSerializer(element):
         rv = []
 
         def serializeElement(element, indent=0):
-            if not(hasattr(element, "tag")):
+            if not hasattr(element, "tag"):
                 element = element.getroot()
             if element.tag == "<!DOCTYPE>":
                 if element.get("publicId") or element.get("systemId"):
diff --git a/html5lib/treewalkers/etree.py b/html5lib/treewalkers/etree.py
index 44653372..411a1d45 100644
--- a/html5lib/treewalkers/etree.py
+++ b/html5lib/treewalkers/etree.py
@@ -37,7 +37,7 @@ def getNodeDetails(self, node):
                 else:
                     node = elt
 
-            if not(hasattr(node, "tag")):
+            if not hasattr(node, "tag"):
                 node = node.getroot()
 
             if node.tag in ("DOCUMENT_ROOT", "DOCUMENT_FRAGMENT"):
diff --git a/requirements-test.txt b/requirements-test.txt
index 57f8f617..8c0ca7c7 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,7 +1,7 @@
 -r requirements.txt
 
 tox>=3.15.1,<4
-flake8>=3.8.1,<3.9
+flake8>=3.8.1,<6
 pytest>=4.6.10,<5 ; python_version < '3'
 pytest>=5.4.2,<7 ; python_version >= '3'
 coverage>=5.1,<6