Drop support for python3.7 and python3.8 and migrate to pyproject.toml

Even Debian oldstable is on python3.9 now, so should be safe, and should also help the typing process that has now started.

Regarding the migration to pyproject.toml, the only thing that seems to be relevant, is that now the helpers are installed under usr/lib/python3.11/site-packages/helpers/ which I guess we should change. I'm unfortunately not exactly sure how.

pmb/__init__.py

@@ -19,8 +19,8 @@ __version__ = "2.2.1"
 
 # Python version check
 version = sys.version_info
-if version < (3, 7):
-    print("You need at least Python 3.7 to run pmbootstrap")
+if version < (3, 9):
+    print("You need at least Python 3.9 to run pmbootstrap")
     print("(You are running it with Python " + str(version.major) +
           "." + str(version.minor) + ")")
     sys.exit()