Postal: Don't try to install cryptography on Python 3.8

The cryptography package is no longer installable on PyPy 3.8, because PyO3
requires Python 3.9. Since there were already problems installing cryptography
on darwin-arm64 for cpython 3.8 (and since cpython 3.8 passed EOL on all platforms
last month), just limit Postal's extra dependency on cryptography to Python >=3.9.

If you are using Postal tracking webhooks with Python 3.8, you'll get an ImportError
unless you also install cryptography somehow (or upgrade to Python 3.9 or later).

CHANGELOG.rst

@@ -25,6 +25,18 @@ Release history
 ^^^^^^^^^^^^^^^
     ..  This extra heading level keeps the ToC from becoming unmanageably long
 
+vNext
+-----
+
+*Unreleased changes*
+
+Breaking changes
+~~~~~~~~~~~~~~~~
+
+* **Postal:** Require Python 3.9 or later for Postal tracking webhook support.
+  (Postal's signature verification uses the "cryptography" package, which is no
+  longer reliably installable with Python 3.8.)
+
 
 v12.0
 -----