text: Title: Use f-strings instead of `str.format`
  Body: Currently, TensorLy uses the format-method for string formatting. However, in Python 3.6, f-strings were introduced, so the only reason to use the format-method instead of f-strings is for support of Python 3.5, which reached end-of-life in 2020. We can therefore safely replace all calls to `str.format` with f-strings, which is more readable. See example below:
  
  ```python
  x = 1.
  s1 = "{}".format(x)
  s2 = f"{x}"
  
  s3 = "{:.1f}".format(x)
  s4 = f"{x:.1f}"
  
  print(s1 == s2)
  print(s3 == s4)
  ```
  ```
  True
  True
  ```
  
  More information is available in this tutorial: https://realpython.com/python-f-strings/