Basic CircleCI (#449)

* basic style checks for circleci

* format

* fix config

.circleci/config.yml

@@ -0,0 +1,35 @@
+version: 2.1
+
+jobs:
+  linux_build_and_test:
+    docker:
+      - image: cimg/python:3.9
+
+    steps:
+      - checkout
+      - run:
+          name: Run style checks
+          command: |
+            pip install pre-commit
+            pre-commit run --all
+            if ! git diff --quiet; then echo 'Style checks failed, please install pre-commit and run pre-commit run --all and push the change'; exit 1; fi
+
+workflows:
+  build_and_test:
+    when:
+      matches:
+        pattern: "^(?!pull/)[-\\w]+$"
+        value: << pipeline.git.branch >>
+    jobs:
+      - linux_build_and_test
+
+  prb:
+    when:
+      matches:
+        pattern: "^pull/\\d+(/head)?$"
+        value: << pipeline.git.branch >>
+    jobs:
+      - hold:
+          type: approval
+      - linux_build_and_test:
+          requires: [ hold ]