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/hookspec.py

@@ -7,12 +7,12 @@ from typing import Mapping
 from typing import Optional
 from typing import Sequence
 from typing import Tuple
+from typing import TYPE_CHECKING
 from typing import Union
 
 import py.path
 from pluggy import HookspecMarker
 
-from _pytest.compat import TYPE_CHECKING
 from _pytest.deprecated import WARNING_CAPTURED_HOOK
 
 if TYPE_CHECKING: