Replace in dict.keys() with in dict (#19)

* Add pattern to simplify `in dict.keys()` to `in dict`

* Rephrase title

.grit/patterns/in_dict_keys.md

@@ -0,0 +1,48 @@
+---
+title: In dict instead of in dict.keys()
+---
+
+Rewrite `in dict.keys()` to `in dict`. Rule [SIM118](https://github.com/MartinThoma/flake8-simplify/issues/40) from [flake8-simplify](https://github.com/MartinThoma/flake8-simplify).
+
+Limitations:
+- The change is not applied to for loops.
+
+
+```grit
+engine marzano(0.1)
+language python
+
+`$var in $dict.keys()` => `$var in $dict`
+```
+
+## Replace `in dict.keys()` with `in dict`
+
+```python
+found = key in foo.keys()
+
+if name in names.keys():
+    print(f"{name} found")
+
+# TODO: this for loop should also be simplified
+for name in names.keys():
+    print(name)
+
+key in sorted(foo.keys())[:10]
+
+(a, b) in foo.items()
+```
+
+```python
+found = key in foo
+
+if name in names:
+    print(f"{name} found")
+
+# TODO: this for loop should also be simplified
+for name in names.keys():
+    print(name)
+
+key in sorted(foo.keys())[:10]
+
+(a, b) in foo.items()
+```