Add dependabot configurations (#29)

This PR adds dependanbot configurations for automatically updating
python dependencies and github actions configurations.

We also add a `.yamllint.yaml` configuration files for yamllint to
suppress certain types of YAML errors.

.github/dependabot.yaml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      python-packages:
+        patterns:
+          - "*"

.yamllint.yaml

@@ -0,0 +1,13 @@
+extends: default
+
+rules:
+  line-length:
+    max: 99
+
+  # Disable checks for truthy values since yamllint reports errors on "on" keywords in GitHub
+  # Actions configuration files (since in YAML 1.1 "on" is a boolean). We can remove this once
+  # yamllint defaults to YAML 1.2 or provides options to specify that.
+  truthy: disable
+
+  # This requires all YAML files begin with "---", which in practice is rarely needed.
+  document-start: disable