py36+: remove TYPE_CHECKING from _pytest.compat

automated with:

```bash
git grep -l 'from .* import TYPE_CHECKING' |
    xargs reorder-python-imports \
        --application-directories .:src \
        --remove-import 'from _pytest.compat import TYPE_CHECKING' \
        --add-import 'from typing import TYPE_CHECKING'
```

src/_pytest/python_api.py

@@ -13,6 +13,7 @@ from typing import Generic
 from typing import Optional
 from typing import Pattern
 from typing import Tuple
+from typing import TYPE_CHECKING
 from typing import TypeVar
 from typing import Union
 
@@ -20,7 +21,6 @@ import _pytest._code
 from _pytest.compat import final
 from _pytest.compat import overload
 from _pytest.compat import STRING_TYPES
-from _pytest.compat import TYPE_CHECKING
 from _pytest.outcomes import fail
 
 if TYPE_CHECKING: