fix ImportError; add a check for it

.github/workflows/test_import.py

@@ -0,0 +1,18 @@
+name: test Python import
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+        with:
+            python-version: '3.x'
+            architecture: 'x64'
+    - run: python -m pip install .
+    - run: python -c 'import dpdata'
+

dpdata/pymatgen/molecule.py

@@ -1,5 +1,8 @@
 import numpy as np
-from pymatgen.core import Molecule
+try:
+    from pymatgen.core import Molecule
+except ImportError:
+    pass
 from collections import Counter
 import dpdata