Blocks: "change <variable> by" block causes import error

The 'change variable by' block causes the following import statement to be emitted:
```from numbers import Number```

Which in turn causes an ImportError on the device.

It also generates rather confusing code:
```
count = (count if isinstance(count, Number) else 0) + -1
```

See the following program as an example of failure:

在編輯器中重現連結的範例，並檢查產生的 Python 中的 import 和變數變更運算式。追蹤該程式區塊的程式碼產生路徑；當範例能在裝置上成功匯入，且產生的程式碼不再產生報告中令人困惑的運算式時，該 issue 就完成了。

由索引模型根據 Issue 內容生成。