Run CI tests against win32 and darwin platform (#3457)

Also use Python 3.8 as default Python version for tests

.travis.yml

@@ -1,6 +1,6 @@
 dist: bionic
 language: python
-python: 3.7
+python: 3.8
 
 jobs:
   include:
@@ -11,11 +11,17 @@ jobs:
     - name: "mypy (typed-ast)"
       python: 3.7
       install: pip install -U git+git://github.com/python/mypy git+git://github.com/python/typed_ast
-      script: ./tests/mypy_test.py
+      script: ./tests/mypy_test.py --platform=linux
     - name: "mypy (ast)"
-      python: 3.8-dev
+      python: 3.8
       install: pip install -U git+git://github.com/python/mypy
-      script: ./tests/mypy_test.py
+      script: ./tests/mypy_test.py --platform=linux
+    - name: "mypy (Windows)"
+      install: pip install -U git+git://github.com/python/mypy
+      script: ./tests/mypy_test.py --platform=win32
+    - name: "mypy (Darwin)"
+      install: pip install -U git+git://github.com/python/mypy
+      script: ./tests/mypy_test.py --platform=darwin
     - name: "mypy self test"
       script: ./tests/mypy_selftest.py
     - name: "check file consistency"

tests/mypy_test.py

@@ -24,6 +24,8 @@ parser.add_argument('-n', '--dry-run', action='store_true', help="Don't actually
 parser.add_argument('-x', '--exclude', type=str, nargs='*', help="Exclude pattern")
 parser.add_argument('-p', '--python-version', type=str, nargs='*',
                     help="These versions only (major[.minor])")
+parser.add_argument('--platform',
+                    help="Run mypy for a certain OS platform (defaults to sys.platform)")
 parser.add_argument('--warn-unused-ignores', action='store_true',
                     help="Run mypy with --warn-unused-ignores "
                     "(hint: only get rid of warnings that are "
@@ -135,6 +137,8 @@ def main():
             flags.append('--disallow-any-generics')
             if args.warn_unused_ignores:
                 flags.append('--warn-unused-ignores')
+            if args.platform:
+                flags.extend(['--platform', args.platform])
             sys.argv = ['mypy'] + flags + files
             if args.verbose:
                 print("running", ' '.join(sys.argv))