Add direct type-check to Travis CI (#24)

...and fix the error found.

.travis.yml

@@ -11,14 +11,20 @@ jobs:
     python: 3.6
     script: |
       set -e
-      export PYTHONPATH=`pwd`/external/mypy
       pytest
   - name: "run test suite with python 3.7"
     python: 3.7
     script: |
       set -e
-      export PYTHONPATH=`pwd`/external/mypy
       pytest
+  - name: "run direct typecheck"
+    python: 3.6
+    script: |
+      set -e
+      mkdir tmpd
+      cp -r sqlalchemy-stubs tmpd/sqlalchemy
+      cd tmpd
+      python3 -m mypy -p sqlalchemy
   - name: "run flake8 on stubs"
     python: 3.6
     script: |
@@ -33,3 +39,4 @@ install: |
   pip install -r external/mypy/test-requirements.txt
   pip install -U .
   python2 -m pip install --user -U .
+  export PYTHONPATH=`pwd`/external/mypy

sqlalchemy-stubs/engine/strategies.pyi

@@ -20,7 +20,7 @@ class ThreadLocalEngineStrategy(DefaultEngineStrategy):
 
 class MockEngineStrategy(EngineStrategy):
     name: str = ...
-    def create(self, name_or_url, executor, **kwargs) -> MockEngineStrategy.MockConnection: ...
+    def create(self, name_or_url, executor, **kwargs) -> MockEngineStrategy.MockConnection: ...  # type: ignore
     class MockConnection(base.Connectable):
         execute: Any = ...
         def __init__(self, dialect, execute) -> None: ...

sqlalchemy-stubs/sql/schema.pyi

@@ -53,7 +53,7 @@ class Column(SchemaItem, ColumnClause, Generic[_T]):
     __visit_name__: str = ...
     key: Any = ...
     primary_key: Any = ...
-    nullable: Any = ...
+    nullable: bool = ...
     default: Any = ...
     server_default: Any = ...
     server_onupdate: Any = ...