Drop Bionic testing

Modern Ansible (and thus Zuul) don't work with python < 3.8. That means
Bionic with Python 3.6 and 3.7 is no longer a viable test platform.
Update Bindep testing to 3.8 on Focal as a result. Also update the
requires-python value to >=3.8 to match.

Change-Id: I8b628d48c5d37ee3fca4b7dc69a8a5c9526b4e09

.zuul.yaml

@@ -31,8 +31,8 @@
         - bindep-ubuntu-noble
         - build-python-release
         - nox-linters
-        - nox-py37:
-            nodeset: ubuntu-bionic
+        - nox-py38:
+            nodeset: ubuntu-focal
         - nox-py312:
             nodeset: ubuntu-noble
         - nox-cover
@@ -42,8 +42,8 @@
         - bindep-ubuntu-noble
         - build-python-release
         - nox-linters
-        - nox-py37:
-            nodeset: ubuntu-bionic
+        - nox-py38:
+            nodeset: ubuntu-focal
         - nox-py312:
             nodeset: ubuntu-noble
         - nox-cover

pyproject.toml

@@ -15,7 +15,6 @@ classifiers = [
     "Operating System :: MacOS :: MacOS X",
     "Operating System :: POSIX :: Linux",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
@@ -48,7 +47,7 @@ keywords = [
 # once support for Python 3.8 is dropped (requires SetupTools>=77).
 name = "bindep"
 readme = {charset = "UTF-8", content-type = "text/x-rst", file = "README.rst"}
-requires-python = ">=3.7"
+requires-python = ">=3.8"
 
 # TODO: replace these with PEP 735 compliant dependency groups once support for
 # Python 3.8 is dropped (requires pip>=25.1).