Update pythonpackage.yml

Attempt to replicate conda install approach from README

.github/workflows/pythonpackage.yml

@@ -1,25 +1,41 @@
 name: Python package
 
-on: [push]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - '*'
 
 jobs:
-  build:
-
-    runs-on: ubuntu-latest
+  conda-build:
+    name: Conda ${{ matrix.python-version }} - ${{ matrix.os }}
+
+    runs-on: ${{ matrix.os }}
     strategy:
+      fail-fast: true
       matrix:
-        python-version: [3.7, 3.8]
+        os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
+        python-version: ['3.7']
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
+    - uses: goanpeca/setup-miniconda@v1.1.2
       with:
+        channels: conda-forge
+        auto-update-conda: true
         python-version: ${{ matrix.python-version }}
+    - name: Install RTree
+      shell: bash -l {0}
+      run: | 
+        conda install -c conda-forge Rtree
     - name: Install dependencies
+      shell: bash -l {0}
       run: |
+        conda activate base
         python -m pip install --upgrade pip
-        pip install numpy meshcat anytree rtree-linux trimesh[easy]
+        pip install numpy meshcat anytree trimesh[easy]
     - name: Test with pytest
       run: |
         pip install pytest pytest-benchmark