text: Title: Scrapy docs: 'make htmlview' does not work
  Body: ### Description
  
  `Path.resolve()` returns the `PosixPath` _class_ so it is not possible to concatenate it with a string.
  
  ```
  webbrowser.open('file://' + Path('build/html/index.html').resolve())
  ```
  
  
  ### Steps to Reproduce
  
  ```
  cd docs/
  make html
  make htmlview
  ```
  
  **Expected behavior:** 
  
  `webbrowser` should open the file `build/html/index.html`
  
  **Actual behavior:**
  
  Get the following error when trying to open the file:
  
  ```
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  TypeError: can only concatenate str (not "PosixPath") to str
  ```
  
  ### Versions
  
  `2.8.0`
  
  ### Possible solution
  
  Instead of trying to concatenate the string `file://` we could use [`as_uri()`](https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.as_uri). For example:
  
  ```
  htmlview: html
  	 $(PYTHON) -c "import webbrowser; from pathlib import Path; \
  	 webbrowser.open(Path('build/html/index.html').resolve().as_uri())"
  ```
  
  
  
  
  
  