Update travis config to use Python 3.8.1.

.travis.yml

@@ -1,14 +1,6 @@
-sudo: false
-language: bash
-os:
-  - linux
-
-addons:
-  apt:
-    packages:
-      - gperf
-      - python
-      - python-serial
+dist: bionic
+language: python
+python: "3.8"
 
 before_install:
   # Save path to the git respository
@@ -29,11 +21,11 @@ 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 release/v3.3 --single-branch --shallow-submodules https://github.com/espressif/esp-idf.git
+  - git clone --recursive --branch v3.3 --single-branch --shallow-submodules 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 -r $IDF_PATH/requirements.txt
+  - pip install --requirement $IDF_PATH/requirements.txt
 
 script:
   # Build project within the original repository