Fix Python version requirement, old references

This requires either a new release or recreation of release and PyPi
distribution, as there is a possibility that dependency resolution may
be affected.

README.md

@@ -26,7 +26,7 @@ RDF fingerprinter may be installed with pip as follows.
 ```
 pip install rdf-fingerprinter
 ```
-Nore that Python version 3.7 or later is required.  
+Note that Python version 3.8 or later is required.
 
 # Usage
 

setup.py

@@ -66,7 +66,7 @@ def find_version(filename):
         "Topic :: Software Development :: Libraries :: Python Modules",
         "Natural Language :: English",
     ],
-    python_requires='>=3.6',
+    python_requires='>=3.8',
     entry_points={
         "console_scripts": ["fingerprint=fingerprint.entrypoints.cli.main:fingerprint_endpoint"]
     }