Travis CI: Remove OS X tests

I can't figure out how to install Python 3 on OS X on Travis.

.travis.yml

@@ -1,8 +1,5 @@
 language: python
 
-env:
-  - PYTHON=python
-
 matrix:
   include:
     - os: linux
@@ -20,26 +17,13 @@ matrix:
         - choco install python --version 3.7
         - export PATH="/c/Python37:/c/Python37/Scripts:$PATH"
         - python -m pip install --upgrade pip wheel
-    - os: osx
-      # Perform the manual steps on osx to install python3
-      language: sh
-      python: 3.7
-      env:
-        - PYTHON=python3
-        - HOMEBREW_NO_INSTALL_CLEANUP=1
-      before_install:
-        # Remove unneeded stuff (which Brew would want to upgrade)
-        - "brew remove numpy gdal postgis postgresql || :"
-        # Install or update Python
-        - "brew install python3 || :"
-        - "brew upgrade python3"
 
 cache:
   directories:
   - $HOME/.cache/pip
 
 install:
-- $PYTHON -m pip install .[dev]
+- python -m pip install .[dev]
 
 script:
-- $PYTHON -m pytest
+- python -m pytest