Drop Python 3.6 and 3.7 support

As of e0baec2 we no longer test the code for Python < 3.8, so it doesn't make sense
to keep "supporting" them

setup.py

@@ -18,12 +18,11 @@
 License :: OSI Approved :: MIT License
 Programming Language :: Python
 Programming Language :: Python :: 3
-Programming Language :: Python :: 3.6
-Programming Language :: Python :: 3.7
 Programming Language :: Python :: 3.8
 Programming Language :: Python :: 3.9
 Programming Language :: Python :: 3.10
 Programming Language :: Python :: 3.11
+Programming Language :: Python :: 3.12
 Operating System :: OS Independent
 Topic :: Software Development :: Libraries :: Python Modules
 Topic :: Software Development :: Documentation
@@ -57,7 +56,7 @@
         ]
     },
     description="A fast and complete Python implementation of Markdown",
-    python_requires=">=3.6, <4",
+    python_requires=">=3.8, <4",
     extras_require=extras_require,
     classifiers=classifiers.strip().split("\n"),
     long_description="""markdown2: A fast and complete Python implementation of Markdown.

test/testall.py

@@ -26,8 +26,8 @@ def _gen_python_names():
     if sys.platform == "win32":
         return
 
-    # generate version numbers from python 3.5 to 3.20
-    for ver in [(3, i) for i in range(5, 20)]:
+    # generate version numbers from python 3.8 to 3.20
+    for ver in [(3, i) for i in range(8, 20)]:
         yield "python%d.%d" % ver
         if sys.platform == "win32":
             yield "python%d%d" % ver
@@ -49,7 +49,7 @@ def _gen_pythons():
 def testall():
     all_warnings = []
     for ver, python in _gen_pythons():
-        if ver < (3, 5):
+        if ver < (3, 8):
             # Don't support Python < 3.5
             continue
         ver_str = "%s.%s" % ver