Use python3.11 in CI to make sure toml is validated

This also fixes a regression from
https://github.com/rust-lang/rust/pull/106085 which stopped testing that
we support python2 in PR CI.

src/bootstrap/bootstrap_test.py

@@ -91,8 +91,7 @@ class GenerateAndParseConfig(unittest.TestCase):
             # Verify this is actually valid TOML.
             tomllib.loads(build.config_toml)
         except ImportError:
-            # too old a version of python
-            pass
+            print("warning: skipping TOML validation, need at least python 3.11", file=sys.stderr)
         return build
 
     def test_no_args(self):

src/ci/docker/host-x86_64/mingw-check-tidy/Dockerfile

@@ -1,6 +1,8 @@
 FROM ubuntu:22.04
 
 ARG DEBIAN_FRONTEND=noninteractive
+# NOTE: intentionally uses python2 for x.py so we can test it still works.
+# validate-toolstate only runs in our CI, so it's ok for it to only support python3.
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
   make \
@@ -8,6 +10,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   file \
   curl \
   ca-certificates \
+  python2.7 \
   python3 \
   python3-pip \
   python3-pkg-resources \
@@ -30,4 +33,4 @@ RUN pip3 install --no-deps --no-cache-dir --require-hashes -r /tmp/reuse-require
 COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
 COPY host-x86_64/mingw-check/validate-error-codes.sh /scripts/
 
-ENV SCRIPT python3 ../x.py test --stage 0 src/tools/tidy tidyselftest
+ENV SCRIPT python2.7 ../x.py test --stage 0 src/tools/tidy tidyselftest

src/ci/docker/host-x86_64/x86_64-gnu-llvm-14/Dockerfile

@@ -2,7 +2,6 @@ FROM ubuntu:22.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 
-# NOTE: intentionally installs both python2 and python3 so we can test support for both.
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
   gcc-multilib \
@@ -11,8 +10,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   file \
   curl \
   ca-certificates \
-  python2.7 \
-  python3 \
+  python3.11 \
   git \
   cmake \
   sudo \