diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 48d77cfdc6f..3773a3213aa 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -13,9 +13,17 @@ repos:
         additional_dependencies:
           - flake8-docstrings==1.3.1
           - pydocstyle==4.0.0
--   repo: https://github.com/pre-commit/mirrors-mypy.git
-    rev: v0.730
+# Using a local "system" mypy instead of the mypy hook, because its
+# results depend on what is installed. And the mypy hook runs in a
+# virtualenv of its own, meaning we'd need to install and maintain
+# another set of our dependencies there... no. Use the "system" one
+# and reuse the environment that is set up anyway already instead.
+-   repo: local
     hooks:
     -   id: mypy
-        args: []
+        name: mypy
+        entry: mypy
+        language: system
+        types: [python]
+        require_serial: true
         exclude: ^script/scaffold/templates/
diff --git a/azure-pipelines-ci.yml b/azure-pipelines-ci.yml
index a566baf6561..257eac57c2d 100644
--- a/azure-pipelines-ci.yml
+++ b/azure-pipelines-ci.yml
@@ -174,12 +174,12 @@ stages:
     steps:
     - template: templates/azp-step-cache.yaml@azure
       parameters:
-        keyfile: 'requirements_test.txt | homeassistant/package_constraints.txt'
+        keyfile: 'requirements_test.txt | setup.py | homeassistant/package_constraints.txt'
         build: |
           python -m venv venv
 
           . venv/bin/activate
-          pip install -r requirements_test.txt -c homeassistant/package_constraints.txt
+          pip install -e . -r requirements_test.txt -c homeassistant/package_constraints.txt
     - script: |
         . venv/bin/activate
         mypy homeassistant
diff --git a/homeassistant/util/ruamel_yaml.py b/homeassistant/util/ruamel_yaml.py
index 6793784abae..b7e8927888c 100644
--- a/homeassistant/util/ruamel_yaml.py
+++ b/homeassistant/util/ruamel_yaml.py
@@ -90,7 +90,7 @@ def load_yaml(fname: str, round_trip: bool = False) -> JSON_TYPE:
     if round_trip:
         yaml = YAML(typ="rt")
         # type ignore: https://bitbucket.org/ruamel/yaml/pull-requests/42
-        yaml.preserve_quotes = True
+        yaml.preserve_quotes = True  # type: ignore
     else:
         if ExtSafeConstructor.name is None:
             ExtSafeConstructor.name = fname