Install typing_extensions on Python 3.6 and 3.7 (#775)

* Install typing_extensions on Python 3.6 and 3.7

* Add CHANGELOG.md

* Black setup.py

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - `ValueError` when trying to decrypt empty metadata values ([#766](https://github.com/pdfminer/pdfminer.six/issues/766))
 - Sphinx errors during building of documentation ([#760](https://github.com/pdfminer/pdfminer.six/pull/760))
 - `TypeError` when getting default width of font ([#720](https://github.com/pdfminer/pdfminer.six/issues/720))
+- Install typing-extensions on Python 3.6 and 3.7 ([#775](https://github.com/pdfminer/pdfminer.six/pull/775))
 - `TypeError` in cmapdb.py when parsing null characters ([#768](https://github.com/pdfminer/pdfminer.six/pull/768))
 
 ### Deprecated

pdfminer/image.py

@@ -7,6 +7,7 @@ from typing import BinaryIO, Tuple
 try:
     from typing import Literal
 except ImportError:
+    # Literal was introduced in Python 3.8
     from typing_extensions import Literal  # type: ignore[misc]
 
 from .jbig2 import JBIG2StreamReader, JBIG2StreamWriter

setup.py

@@ -19,6 +19,7 @@ setup(
     install_requires=[
         "charset-normalizer >= 2.0.0",
         "cryptography >= 36.0.0",
+        'typing_extensions; python_version < "3.8"',
     ],
     extras_require={
         "dev": ["pytest", "nox", "black", "mypy == 0.931"],