Style fix and docstring for the APM QA Python tool

webrtc/modules/audio_processing/test/py_quality_assessment/README.md

 # APM Quality Assessment tool
 
 Python wrapper of `audioproc_f` with which quality assessment can be
 automatized. The tool allows to simulate different noise conditions, input
 signals, APM configurations and it computes different scores.
 Once the scores are computed, the results can be easily exported to an HTML page
 which allows to listen to the APM input and output signals and also the
 reference one used for evaluation.
 
 ## Dependencies
 
  - OS: Linux
  - Python 2.7
  - Python libraries: numpy, scipy, pydub (0.17.0+)
-   - `$ sudo apt install python-numpy python-scipy`
-   - `$ sudo pip install pydub`
+ - It is recommended that a dedicated Python environment is used
+   - install `virtualenv`
+   - `$ sudo apt-get install python-virtualenv`
+   - setup a new Python environment (e.g., `my_env`)
+   - `$ cd ~ && virtualenv my_env`
+   - activate the new Python environment
+   - `$ source ~/my_env/bin/activate`
+   - add dependcies via `pip`
+   - `(my_env)$ pip install numpy pydub scipy`
  - PolqaOem64 (see http://www.polqa.info/)
     - Tested with POLQA Library v1.180 / P863 v2.400
  - Aachen Impulse Response (AIR) Database
     - Download https://www2.iks.rwth-aachen.de/air/air_database_release_1_4.zip
  - Input probing signals and noise tracks (you can make your own dataset - *1)
 
 ## Build
 
  - Compile WebRTC
  - Go to `out/Default/py_quality_assessment` and check that
@@ skipping 64 matching lines @@
 Depending on the license, the POLQA tool may take “breaks” as a way to limit the
 throughput. When this happens, the APM Quality Assessment tool is slowed down.
 For more details about this limitation, check Section 10.9.1 in the POLQA manual
 v.1.18.
 
 In case of issues with the POLQA score computation, check
 `py_quality_assessment/eval_scores.py` and adapt
 `PolqaScore._parse_output_file()`.
 The code can be also fixed directly into the build directory (namely,
 `out/Default/py_quality_assessment/eval_scores.py`).