PY-20744 Parse PEP-526 variable annotations

Annotation is preserved at the level of assignment nodes similar to
where CPython keeps them in its AST (in special "augassign" nodes).
For type annotations in form "x: int" without variable initialization
special statement PyTypeDefinitionStatement was introduced.

python/testData/psi/VariableAnnotations.py

@@ -0,0 +1,8 @@
+class C:
+    x: int
+    y: None = 42
+
+    def m(self, d):
+        x: List[bool]
+        d['foo']: str
+        (d['bar']): float

python/testData/psi/VariableAnnotations.txt

@@ -0,0 +1,93 @@
+PyFile:VariableAnnotations.py
+  PyClass: C
+    PsiElement(Py:CLASS_KEYWORD)('class')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('C')
+    PyArgumentList
+      <empty list>
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyTypeDeclarationStatement
+        PyTargetExpression: x
+          PsiElement(Py:IDENTIFIER)('x')
+        PyAnnotation
+          PsiElement(Py:COLON)(':')
+          PsiWhiteSpace(' ')
+          PyReferenceExpression: int
+            PsiElement(Py:IDENTIFIER)('int')
+      PsiWhiteSpace('\n    ')
+      PyAssignmentStatement
+        PyTargetExpression: y
+          PsiElement(Py:IDENTIFIER)('y')
+        PyAnnotation
+          PsiElement(Py:COLON)(':')
+          PsiWhiteSpace(' ')
+          PyNoneLiteralExpression
+            PsiElement(Py:NONE_KEYWORD)('None')
+        PsiWhiteSpace(' ')
+        PsiElement(Py:EQ)('=')
+        PsiWhiteSpace(' ')
+        PyNumericLiteralExpression
+          PsiElement(Py:INTEGER_LITERAL)('42')
+      PsiWhiteSpace('\n\n    ')
+      PyFunction('m')
+        PsiElement(Py:DEF_KEYWORD)('def')
+        PsiWhiteSpace(' ')
+        PsiElement(Py:IDENTIFIER)('m')
+        PyParameterList
+          PsiElement(Py:LPAR)('(')
+          PyNamedParameter('self')
+            PsiElement(Py:IDENTIFIER)('self')
+          PsiElement(Py:COMMA)(',')
+          PsiWhiteSpace(' ')
+          PyNamedParameter('d')
+            PsiElement(Py:IDENTIFIER)('d')
+          PsiElement(Py:RPAR)(')')
+        PsiElement(Py:COLON)(':')
+        PsiWhiteSpace('\n        ')
+        PyStatementList
+          PyTypeDeclarationStatement
+            PyTargetExpression: x
+              PsiElement(Py:IDENTIFIER)('x')
+            PyAnnotation
+              PsiElement(Py:COLON)(':')
+              PsiWhiteSpace(' ')
+              PySubscriptionExpression
+                PyReferenceExpression: List
+                  PsiElement(Py:IDENTIFIER)('List')
+                PsiElement(Py:LBRACKET)('[')
+                PyReferenceExpression: bool
+                  PsiElement(Py:IDENTIFIER)('bool')
+                PsiElement(Py:RBRACKET)(']')
+          PsiWhiteSpace('\n        ')
+          PyTypeDeclarationStatement
+            PySubscriptionExpression
+              PyReferenceExpression: d
+                PsiElement(Py:IDENTIFIER)('d')
+              PsiElement(Py:LBRACKET)('[')
+              PyStringLiteralExpression: foo
+                PsiElement(Py:SINGLE_QUOTED_STRING)(''foo'')
+              PsiElement(Py:RBRACKET)(']')
+            PyAnnotation
+              PsiElement(Py:COLON)(':')
+              PsiWhiteSpace(' ')
+              PyReferenceExpression: str
+                PsiElement(Py:IDENTIFIER)('str')
+          PsiWhiteSpace('\n        ')
+          PyTypeDeclarationStatement
+            PyParenthesizedExpression
+              PsiElement(Py:LPAR)('(')
+              PySubscriptionExpression
+                PyReferenceExpression: d
+                  PsiElement(Py:IDENTIFIER)('d')
+                PsiElement(Py:LBRACKET)('[')
+                PyStringLiteralExpression: bar
+                  PsiElement(Py:SINGLE_QUOTED_STRING)(''bar'')
+                PsiElement(Py:RBRACKET)(']')
+              PsiElement(Py:RPAR)(')')
+            PyAnnotation
+              PsiElement(Py:COLON)(':')
+              PsiWhiteSpace(' ')
+              PyReferenceExpression: float
+                PsiElement(Py:IDENTIFIER)('float')