Relax lowercase condition in N806 (#562)

resources/test/fixtures/N806.py

@@ -2,3 +2,4 @@ def f():
     lower = 0
     Camel = 0
     CONSTANT = 0
+    _ = 0

src/pep8_naming/checks.rs

@@ -118,7 +118,7 @@ pub fn non_lowercase_variable_in_function(scope: &Scope, expr: &Expr, name: &str
     if !matches!(scope.kind, ScopeKind::Function(FunctionScope { .. })) {
         return None;
     }
-    if !is_lower(name) {
+    if name.to_lowercase() != name {
         return Some(Check::new(
             CheckKind::NonLowercaseVariableInFunction(name.to_string()),
             Range::from_located(expr),