Remove unused dependency on sortedcontainers (#525)

* Remove unused sortedcontainers package

* Fix changelog format

* Fix a link to the PR

* Update CHANGELOG.md

Co-authored-by: Pieter Marsman <pietermarsman@gmail.com>

CHANGELOG.md

@@ -3,6 +3,11 @@ All notable changes in pdfminer.six will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [Unreleased]
+
+### Removed
+- Unused dependency on `sortedcontainers` package ([#525](https://github.com/pdfminer/pdfminer.six/pull/525))
+
 ## [20201018]
 
 ### Deprecated

setup.py

@@ -15,7 +15,6 @@ setup(
     install_requires=[
         'chardet ; python_version > "3.0"',
         'cryptography',
-        'sortedcontainers',
     ],
     extras_require={
         "dev": ["nose", "tox"],