Remove duplicate requirements files

Author: mherrmann

docs/README.md

@@ -10,7 +10,7 @@ Ensure you have `python` and `pip` installed on your
 system and then run this command in the project root:
 
 ```bash
-pip install -r requirements-dev.txt
+pip install -e ".[docs]"
 make -C docs/ html
 ```
 

requirements-dev.txt

@@ -37,5 +37,8 @@
 	tests_require=[
 		'psutil==5.6.6',
 		"pywin32 >= 227;platform_system=='Windows'"
-	]
+	],
+	extras_require={
+        'docs': ['sphinx-rtd-theme==0.5.0', 'sphinx==3.2.1'],
+    }
 )