Bumping up to Python 3.8

Author: rvanren

Makefile

@@ -21,7 +21,7 @@ type-check:
 		--follow-imports silent \
 		--check-untyped-defs \
 		--allow-untyped-defs \
-		--python-version 3.6 \
+		--python-version 3.8 \
 		--html-report mypy-report \
 		--txt-report mypy-report
 

setup.py

@@ -30,8 +30,8 @@ def get_version(rel_path: str) -> str:
 PROJECT_DIR_NAME = 'harmony_model_checker'
 
 python_version = sys.version_info[:2]
-if python_version < (3, 6):
-    print("{} requires Python version 3.6 or later".format(PACKAGE_NAME))
+if python_version < (3, 8):
+    print("{} requires Python version 3.8 or later".format(PACKAGE_NAME))
     print("(Version {}.{} detected)".format(*python_version))
     sys.exit(1)
 
@@ -161,7 +161,7 @@ def get_c_extension_include_dirs():
             'harmony=harmony_model_checker.main:main'
         ]
     },
-    python_requires=">=3.6",
+    python_requires=">=3.8",
     ext_modules=[module],
     cmdclass={
         "build_ext": BuildExtCommand