Handle variables defined only under a type checking clause

These kind of variables will cause a runtime error, since the variable
will not be defined, unless we use the `annotations` future.

Close #3081

pylint/checkers/variables.py

@@ -1418,6 +1418,11 @@ def _is_variable_violation(
                     # Single statement function, with the statement on the
                     # same line as the function definition
                     maybee0601 = False
+            if isinstance(defstmt, (astroid.Import, astroid.ImportFrom)):
+                defstmt_parent = defstmt.parent
+                if isinstance(defstmt_parent, astroid.If):
+                    if defstmt_parent.test.as_string() in TYPING_TYPE_CHECKS_GUARDS:
+                        maybee0601 = True
 
         return maybee0601, annotation_return, use_outer_definition
 

tests/functional/u/undefined_variable.py

@@ -245,3 +245,14 @@ def onclick(event):
     fig = plt.figure()
     fig.canvas.mpl_connect('button_press_event', onclick)
     plt.show(block=True)
+
+
+# pylint: disable=wrong-import-position
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from datetime import datetime
+
+
+def func_should_fail(_dt: datetime):  # [used-before-assignment]
+    pass

tests/functional/u/undefined_variable.txt

@@ -24,3 +24,4 @@ undefined-variable:161::Undefined variable 'unicode_2'
 undefined-variable:171::Undefined variable 'unicode_4'
 undefined-variable:226:LambdaClass4.<lambda>:Undefined variable 'LambdaClass4'
 undefined-variable:234:LambdaClass5.<lambda>:Undefined variable 'LambdaClass5'
+used-before-assignment:257:func_should_fail:Using variable 'datetime' before assignment