Prepare for release v3.8.0

Prepare the API changes

Add some text about type hints

Update dead links to ipython, nbviewer, and cairocffi

Update versions in switcher, security, release notes and ghstats

Author: ksunden

SECURITY.md

@@ -1,14 +1,19 @@
 [
     {
-        "name": "3.7 (stable)",
+        "name": "3.8 (stable)",
         "version": "stable",
         "url": "https://matplotlib.org/stable/"
     },
     {
-        "name": "3.8 (dev)",
+        "name": "3.9 (dev)",
         "version": "dev",
         "url": "https://matplotlib.org/devdocs/"
     },
+    {
+        "name": "3.7",
+        "version": "3.7.3",
+        "url": "https://matplotlib.org/3.7.3/"
+    },
     {
         "name": "3.6",
         "version": "3.6.3",