Run black on all the notebooks

- we use black's default settings
- some cells are NOT kept in black's format to:
  - increase readability
  - or show Python's flexibility with regard to style

00_python_in_a_nutshell/00_content_arithmetic.ipynb

@@ -123,7 +123,7 @@
     }
    ],
    "source": [
-    "2 ** 3"
+    "2**3"
    ]
   },
   {
@@ -143,7 +143,7 @@
     }
    ],
    "source": [
-    "2 * 2 ** 3"
+    "2 * 2**3"
    ]
   },
   {

00_python_in_a_nutshell/05_content_functions.ipynb

@@ -385,7 +385,7 @@
    ],
    "source": [
     "for number in numbers:\n",
-    "    square = number ** 2\n",
+    "    square = number**2\n",
     "    print(\"The square of\", number, \"is\", square)"
    ]
   },