MNT: Migrate Travis CI to GitHub Actions (#163)

* MNT: Migrate Travis CI to GitHub Actions

Travis has constant backlog and its build sometimes fails to fire.
Long live GH Actions! :/

* Copy deploying tags docs into a separate workflow

* fix yml: https://github.com/kernc/backtesting.py/actions/runs/332715564

* fix docs build with -e

* pip install wheel

* it's late

* Make test type a parameter, becasuse it's shown in the UI

* Remove Py3.9 testing: It's new and weird

https://github.com/kernc/backtesting.py/runs/1318477083

* Fetch tags for correct __version__

* test deploy-docs workflow ...

* mark .github/deploy-gh-pages.sh executable

* give 'fetch tags' step a name

* goddamn debug

* set github-actions git credentials

* undo 'test deploy-docs workflow'

* DOC: Update 'Build: Passing' link in README

* MNT: Bump minimal Python to 3.6+ in setup.py

* DOC: Bump mentions to Python 3.6+

* move lint deps to setup.py [dev] group

setup.py

@@ -1,8 +1,8 @@
 import os
 import sys
 
-if sys.version_info < (3, 4):
-    sys.exit('ERROR: Backtesting.py requires Python 3.4+')
+if sys.version_info < (3, 6):
+    sys.exit('ERROR: Backtesting.py requires Python 3.6+')
 
 
 if __name__ == '__main__':
@@ -31,7 +31,6 @@ if __name__ == '__main__':
             'write_to': os.path.join('backtesting', '_version.py'),
         },
         install_requires=[
-            'typing ; python_version < "3.5"',
             'numpy',
             'pandas >= 0.25.0, != 0.25.0',
             'bokeh >= 1.4.0',
@@ -51,10 +50,12 @@ if __name__ == '__main__':
             ],
             'dev': [
                 'flake8',
+                'coverage',
+                'mypy',
             ],
         },
         test_suite="backtesting.test",
-        python_requires='>=3.4',
+        python_requires='>=3.6',
         author='Zach Lûster',
         classifiers=[
             'Intended Audience :: Financial and Insurance Industry',