Attempt to replace Travis CI with GH Actions

Publishing is broken in Travis and I don't think there's much point in
trying to fix something in an EOL platform.

.github/workflows/ci.yml

@@ -12,3 +12,10 @@ jobs:
       - uses: actions/setup-python@v1
       - run: tox
       - run: bash <(curl -s https://codecov.io/bash) -f .tox/py3-testcoverage.xml -f .tox/py27-testcoverage.xml
+      - run: python3 -m pip install build --user
+      - run: python3 -m build --sdist --wheel --outdir dist/ .
+      - name: Publish distribution to PyPI
+        if: startsWith(github.ref, 'refs/tags')
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          password: ${{ secrets.PYPI_API_TOKEN }}

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/ADTRAN/netconf_client.svg?branch=master)](https://travis-ci.org/ADTRAN/netconf_client)
+![Build Status](https://github.com/ADTRAN/netconf_client/workflows/CI%20Checks/badge.svg)
 [![PyPI version](https://badge.fury.io/py/netconf-client.svg)](https://badge.fury.io/py/netconf-client)
 [![Documentation Status](https://readthedocs.org/projects/netconf-client/badge/?version=latest)](https://netconf-client.readthedocs.io/en/latest/?badge=latest)
 [![codecov](https://codecov.io/gh/ADTRAN/netconf_client/branch/master/graph/badge.svg)](https://codecov.io/gh/ADTRAN/netconf_client)