Remove python2 support from CI, _six, etc. (#370)

* Remove python2 step 1

* badge

* Delete _six.py

.github/workflows/python2.yml

-name: python2
-
-on:
-  pull_request:
-  push:
-    branches:
-      - master
-  schedule:
-    - cron: '0 0 * * *'
-
-jobs:
-  build:
-
-    runs-on: ubuntu-latest
-    strategy:
-      max-parallel: 4
-      matrix:
-        python-version: [2.7]
-
-    steps:
-    - uses: actions/checkout@v1
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
-      with:
-        python-version: ${{ matrix.python-version }}
-    - name: Install dependencies
-      run: |
-        ci/install_dependencies_python2.sh
-        pip install .
-    - name: Run minimum python2 tests
-      run: |
-        python2 examples/energy_force.py
-        python2 examples/ase_interface.py

README.md

@@ -9,7 +9,6 @@ Checks:
 
 [![Actions Status](https://github.com/aiqm/torchani/workflows/docs/badge.svg)](https://github.com/aiqm/torchani/actions)
 [![Actions Status](https://github.com/aiqm/torchani/workflows/flake8/badge.svg)](https://github.com/aiqm/torchani/actions)
-[![Actions Status](https://github.com/aiqm/torchani/workflows/python2/badge.svg)](https://github.com/aiqm/torchani/actions)
 [![Actions Status](https://github.com/aiqm/torchani/workflows/runnable%20submodules/badge.svg)](https://github.com/aiqm/torchani/actions)
 [![Actions Status](https://github.com/aiqm/torchani/workflows/tools/badge.svg)](https://github.com/aiqm/torchani/actions)
 [![Actions Status](https://github.com/aiqm/torchani/workflows/unit%20tests/badge.svg)](https://github.com/aiqm/torchani/actions)

ci/install_dependencies_python2.sh

-#!/bin/bash
-
-python -m pip install --upgrade pip
-pip install --pre torch torchvision -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html
-pip install tqdm pyyaml future pkbar
-pip2 install 'ase<=3.17'

torchani/_six.py

-import math
-
-
-if not hasattr(math, 'inf'):
-    math.inf = float('inf')

torchani/aev.py

-from __future__ import division
 import torch
-from . import _six  # noqa:F401
 import math
 from typing import Tuple, Optional
 

torchani/ase.py

@@ -5,7 +5,6 @@
     https://wiki.fysik.dtu.dk/ase
 """
 
-from __future__ import absolute_import
 import torch
 from .nn import Sequential
 import ase.neighborlist

torchani/neurochem/__init__.py

@@ -9,7 +9,6 @@ import struct
 import itertools
 import math
 import timeit
-from . import _six  # noqa:F401
 import collections
 import sys
 from ..nn import ANIModel, Ensemble, Gaussian, Sequential

torchani/neurochem/_six.py

-import collections
-
-
-if not hasattr(collections, 'abc'):
-    collections.abc = collections