enable pip/pre-commit caching on travis and appveyor

.travis.yml

@@ -77,3 +77,7 @@ notifications:
     skip_join: true
   email:
     - pytest-commit@python.org
+cache:
+    directories:
+        - $HOME/.cache/pip
+        - $HOME/.cache/pre-commit

appveyor.yml

@@ -42,3 +42,7 @@ build: false  # Not a C# project, build stuff at the test step instead.
 
 test_script:
   - call scripts\call-tox.bat
+
+cache:
+  - '%LOCALAPPDATA%\pip\cache'
+  - '%USERPROFILE%\.cache\pre-commit'

changelog/3502.trivial.rst

@@ -0,0 +1 @@
+Enable caching for pip/pre-commit in order to reduce build time on travis/appveyor.