Add Appveyor to Python 2.7 branch (#1434)

* Add Appveyor to Python 2.7 branch

* Adapt AppVeyor config for Python 2

* Replace -j0 with -j2: -j0 is ignored on Python 2
* Replace --slowest with --slow
* Remove --timeout option (it was introduced in Python 3)

.github/appveyor.yml

+version: 2.7.13+.{build}
+clone_depth: 5
+build_script:
+- cmd: PCbuild\build.bat -e
+test_script:
+- cmd: PCbuild\rt.bat -q -uall -rwW --slow -j2
+
+# Only trigger AppVeyor if actual code or its configuration changes
+only_commits:
+  files:
+    - .github/appveyor.yml
+    - .gitattributes
+    - Grammar/
+    - Include/
+    - Lib/
+    - Modules/
+    - Objects/
+    - PC/
+    - PCBuild/
+    - Parser/
+    - Programs/
+    - Python/
+    - Tools/