Update Travis config to build with Python3.

Author: DavidAntliff

.travis.yml

@@ -1,7 +1,9 @@
 dist: bionic
 env:
   global:
+    # Set Python 3.7.1 as the default python
     PATH=/opt/python/3.7.1/bin:$PATH
+language: bash
 
 addons:
   apt:
@@ -29,15 +31,13 @@ install:
   # Make xtensa-esp32-elf available for all terminal sessions
   - export PATH=$PATH:$HOME/esp/xtensa-esp32-elf/bin
   # Get ESP-IDF from github
-  - git clone --recursive --branch v3.3 https://github.com/espressif/esp-idf.git
+  - git clone --recursive --branch v3.3 --single-branch https://github.com/espressif/esp-idf.git
   # Set the path to ESP-IDF directory
   - export IDF_PATH=~/esp/esp-idf
   # Install python dependencies
   - pip install --user --requirement $IDF_PATH/requirements.txt
 
 script:
-  - which python
-  - python --version
   # Go back to the git repository
   - cd $PROJECT_PATH
   # Update configuration so that kconfig doesn't start interactive mode