Converge supported Python onto 3.9 (MR 2326)

Python 3.9 was required in pmb/__init__.py already, but some references
still said 3.8 and 3.7.

Co-Developed-By: Oliver Smith <ollieparanoid@postmarketos.org>

.ci/vermin.sh

@@ -10,7 +10,7 @@ fi
 
 # shellcheck disable=SC2046
 vermin \
-	-t=3.7- \
+	-t=3.9- \
 	--backport argparse \
 	--backport configparser \
 	--backport enum \

README.md

@@ -43,7 +43,7 @@ Issues are being tracked
   * [Linux kernel 3.17 or higher](https://postmarketos.org/oldkernel)
   * Note: kernel versions between 5.8.8 and 6.0 might 
     [have issues with parted](https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2309).
-* Python 3.7+
+* Python 3.9+
 * OpenSSL
 * git
 * ps

docs/usage.rst

@@ -41,7 +41,7 @@ pmbootstrap requires the following:
        Kernel version 5.8 - 6.0 might have issues with loop-devices
 
 
-  * Python 3.7+
+  * Python 3.9+
   * OpenSSL
   * git
   * ps

pyproject.toml

@@ -30,8 +30,8 @@ exclude = ["aports", "docs", "keys", "test", "test.pmb_test"]
 
 [tool.ruff]
 line-length=100
-# Assume Python 3.8
-target-version = "py38"
+# Assume Python 3.9
+target-version = "py39"
 # E402: module import not on top of file, not possible for testcases
 # E722: do not use bare except
 lint.ignore=["E402", "E722"]