ci: re-introduce CircleCI with updated implementation

Signed-off-by: Paul Horton <phorton@sonatype.com>

.circleci/circleci-readme.md

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+CI Debug Notes
+================
+To validate some circleci stuff, I was able to run a “build locally” using the steps below.
+The local build runs in a docker container.
+
+  * (Once) Install circleci client (`brew install circleci`)
+
+  * Convert the “real” config.yml into a self contained (non-workspace) config via:
+
+        circleci config process .circleci/config.yml > .circleci/local-config.yml
+
+  * Run a local build with the following command:
+
+        circleci local execute -c .circleci/local-config.yml --job 'build'
+
+    Typically, both commands are run together:
+
+        circleci config process .circleci/config.yml > .circleci/local-config.yml && circleci local execute -c .circleci/local-config.yml --job 'build'
+
+    With the above command, operations that cannot occur during a local build will show an error like this:
+
+      ```
+      ... Error: FAILED with error not supported
+      ```
+
+      However, the build will proceed and can complete “successfully”, which allows you to verify scripts in your config, etc.
+
+      If the build does complete successfully, you should see a happy yellow `Success!` message.

.circleci/config.yml

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+# Copyright 2019-present Sonatype Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+version: 2.1
+
+commands:
+  ensure_poetry_installed:
+    description: "Installs Poetry ready for use"
+    steps:
+      - run: |
+          python -m ensurepip --default-pip
+          pip install --upgrade pip
+          pip install poetry
+
+executors:
+  python39:
+    docker:
+      - image: cimg/python:3.9
+  python38:
+    docker:
+      - image: cimg/python:3.8
+  python37:
+    docker:
+      - image: cimg/python:3.7
+  python36:
+    docker:
+      - image: cimg/python:3.6
+
+jobs:
+  build_and_test:
+    parameters:
+      python_version:
+        type: string
+    executor: python<< parameters.python_version >>
+    steps:
+      - ensure_poetry_installed
+      - checkout
+      - restore_cache: # ensure this step occurs *before* installing dependencies
+          name: "Restore any valid cache"
+          key: dependencies-{{ .Branch }}-{{ checksum "poetry.lock" }}
+      - run:
+          command: |
+            poetry install
+      - save_cache:
+          name: "Cache dependencies"
+          key: dependencies-{{ .Branch }}-{{ checksum "poetry.lock" }}
+          paths:
+            - /home/circleci/.cache/pypoetry/virtualenvs
+      - run:
+          command: |
+            poetry build
+      - run:
+          command: |
+            poetry run tox -e py<< parameters.python_version >>
+
+  coding_standards:
+    executor: python39
+    steps:
+      - ensure_poetry_installed
+      - checkout
+      - restore_cache: # ensure this step occurs *before* installing dependencies
+          name: "Restore any valid cache"
+          key: dependencies-{{ .Branch }}-{{ checksum "poetry.lock" }}
+      - run:
+          command: |
+            poetry install
+      - save_cache:
+          name: "Cache dependencies"
+          key: dependencies-{{ .Branch }}-{{ checksum "poetry.lock" }}
+          paths:
+            - /home/circleci/.cache/pypoetry/virtualenvs
+      - run:
+          command: |
+            poetry run tox -eflake8
+
+  release_and_pypi_publish:
+    executor: python39
+    environment:
+      CIRCLECI: "true"
+      GH_TOKEN: None
+    steps:
+      - ensure_poetry_installed
+      - run:
+          name: "Install python-semantic-release"
+          command: |
+            export
+            pip install python-semantic-release
+      - checkout
+      - restore_cache: # ensure this step occurs *before* installing dependencies
+          name: "Restore any valid cache"
+          key: dependencies-{{ .Branch }}-{{ checksum "poetry.lock" }}
+      - run:
+          name: "Build for release"
+          command: |
+            poetry install
+            poetry build
+      - run:
+          name: "Semantic Release"
+          command: |
+            git stash -u
+            git fetch && git reset --hard origin/main
+            semantic-release --noop publish
+
+workflows:
+  cicd:
+    jobs:
+      - build_and_test:
+          matrix:
+            parameters:
+              python_version: ["39", "38", "37", "36"]
+      - coding_standards
+  release:
+    jobs:
+      - release_and_pypi_publish