Setup coverage testing, closes kivy#1058

Also enables report to coveralls.io via GitHub Action.
Note the `COVERALLS_REPO_TOKEN` was setup via:
https://github.com/kivy/buildozer/settings/secrets
And is accessible for repos admin via:
https://coveralls.io/github/kivy/buildozer

.github/workflows/test_python.yml

@@ -25,11 +25,15 @@ jobs:
 
     - name: Requirements
       run: |
-        pip install --timeout=120 -U setuptools tox>=2.0
+        pip install -U coveralls setuptools tox>=2.0
         tox
     - name: Build
       run: |
         docker build --tag=buildozer .
     - name: Test
       run: |
         docker run buildozer --version
+    - name: coveralls
+      run: coveralls
+      env:
+        COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}

README.md

@@ -2,6 +2,7 @@ Buildozer
 =========
 
 [![Build](https://github.com/kivy/buildozer/workflows/Continuous%20Integration/badge.svg)](https://github.com/kivy/buildozer/actions?query=workflow%3A%22Continuous+Integration%22)
+[![Coverage Status](https://coveralls.io/repos/github/kivy/buildozer/badge.svg)](https://coveralls.io/github/kivy/buildozer)
 [![Backers on Open Collective](https://opencollective.com/kivy/backers/badge.svg)](#backers)
 [![Sponsors on Open Collective](https://opencollective.com/kivy/sponsors/badge.svg)](#sponsors)
 

tox.ini

@@ -5,8 +5,15 @@ envlist = pep8,py27,py3
 deps =
     mock
     pytest
+    py3: coveralls
 commands = pytest tests/
 
+[testenv:py3]
+# for py3 env we will get code coverage
+commands =
+    coverage run --branch --source=buildozer -m pytest {posargs:tests/}
+    coverage report -m
+
 [testenv:pep8]
 deps = flake8
 commands = flake8 buildozer/