diff --git a/.travis.yml b/.travis.yml
index a1ec881..34cf5b2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,6 @@ python:
   - "3.8"
   - "3.9"
 install:
-  - pip install tox==3.14.0 tox-travis flake8
+  - pip install tox tox-travis
 script:
   - tox -r
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 29059ac..8b13686 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Support for Python 3.4 and 3.5 ([#522](https://github.com/pdfminer/pdfminer.six/pull/522))
 - Unused dependency on `sortedcontainers` package ([#525](https://github.com/pdfminer/pdfminer.six/pull/525))
 - Support for non-standard output streams that are not binary ([#523](https://github.com/pdfminer/pdfminer.six/pull/523))
+- Dependency on typing-extensions introduced by [#661](https://github.com/pdfminer/pdfminer.six/pull/661) ([#677](https://github.com/pdfminer/pdfminer.six/pull/677))
 
 ## [20201018]
 
diff --git a/pdfminer/utils.py b/pdfminer/utils.py
index a5cf033..d483a48 100644
--- a/pdfminer/utils.py
+++ b/pdfminer/utils.py
@@ -7,7 +7,6 @@ import struct
 from typing import (Any, BinaryIO, Callable, Dict, Generic, Iterable, Iterator,
                     List, Optional, Set, TextIO, Tuple, TypeVar, Union,
                     TYPE_CHECKING, cast)
-from typing_extensions import Literal
 from html import escape
 
 if TYPE_CHECKING:
@@ -55,10 +54,10 @@ class open_filename(object):
         exc_type: object,
         exc_val: object,
         exc_tb: object
-    ) -> Literal[False]:
+    ) -> None:
         if self.closing:
             self.file_handler.close()
-        return False
+        return
 
 
 def make_compat_bytes(in_str: str) -> bytes:
diff --git a/tools/pdf2txt.py b/tools/pdf2txt.py
index 47e2c79..8f7896c 100755
--- a/tools/pdf2txt.py
+++ b/tools/pdf2txt.py
@@ -5,7 +5,6 @@ import argparse
 import logging
 import sys
 from typing import Any, Container, Iterable, List, Optional, Union
-from typing_extensions import Literal
 
 import pdfminer.high_level
 from pdfminer.layout import LAParams
@@ -18,7 +17,7 @@ OUTPUT_TYPES = ((".htm", "html"),
                 (".xml", "xml"),
                 (".tag", "tag"))
 
-FloatOrDisabled = Union[float, Literal["disabled"]]
+FloatOrDisabled = Union[float, str]  # Union[float, Literal["disabled"]]
 
 
 def float_or_disabled(x: str) -> FloatOrDisabled:
diff --git a/tox.ini b/tox.ini
index 2a25d50..af5d36f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,15 +1,33 @@
 [tox]
-envlist = py{36,37,38,3.9}
+envlist = py{36,37,38,39}-{nose,flake8,mypy,docs}
 
-[testenv]
-extras =
-    dev
-    docs
-whitelist_externals =
+[testenv:py{36,37,38,39}-nose]
+deps =
+    nose
+allowlist_externals =
+    nosetests
+commands =
+    nosetests --nologcapture
+
+[testenv:py{36,37,38,39}-flake8]
+deps =
+    flake8
+allowlist_externals =
     flake8
 commands =
     flake8 pdfminer/ tools/ tests/ --count --statistics
+
+[testenv:py{36,37,38,39}-mypy]
+deps =
+    mypy
+allowlist_externals =
+    mypy
+commands =
     mypy --install-types --non-interactive --show-error-codes .
-    nosetests --nologcapture
+
+[testenv:py{36,37,38,39}-docs]
+extras =
+    docs
+commands =
     python -m sphinx -b html docs/source docs/build/html
     python -m sphinx -b doctest docs/source docs/build/doctest