Fix tox in travis, remove --pre option in tox.ini

Author: Cito

.travis.yml

@@ -19,7 +19,7 @@ matrix:
        python: 3.8
     -  env: TOXENV=mypy
        python: 3.8
-install: pip install coveralls tox
+install: pip install "tox==3.15.0" "coveralls==1.11.1"
 script: tox
 after_success: coveralls
 cache:

tox.ini

@@ -2,7 +2,7 @@
 envlist = py{27,35,36,37,py38,py,py3},pre-commit,mypy,docs
 
 [testenv]
-install_command = python -m pip install --ignore-installed --pre {opts} {packages}
+install_command = python -m pip install --ignore-installed {opts} {packages}
 deps =
     .[test]
 commands =