Tests and Changelog for basefee.

test/libsolidity/syntaxTests/viewPureChecker/basefee_not_pure.sol

@@ -0,0 +1,13 @@
+contract C {
+    function f() public pure {
+        assembly { pop(basefee()) }
+    }
+    function g() public pure returns (uint) {
+        return block.basefee;
+    }
+}
+// ====
+// EVMVersion: >=london
+// ----
+// TypeError 2527: (67-76): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".
+// TypeError 2527: (147-160): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".