Fix raise from compatibility

Pipfile

@@ -9,6 +9,7 @@ flask-wtf = "*"
 flask-babel = "*"
 flask-bootstrap-4-alpha = {ref = "fork", git = "https://github.com/NyanKiyoshi/flask-bootstrap-4-alpha"}
 keyboard = "*"
+six = "*"
 
 [dev-packages]
 pytest = "*"

README.md

@@ -1,4 +1,20 @@
-# Texrrow
+<h1 align="center">Texrrow</h1>
+
+<p align="center">
+    <a href="https://pypi.org/project/texrrow/">
+        <img src="https://img.shields.io/pypi/v/texrrow.svg" alt="PyPi Release" />
+    </a>
+    <a href="#">
+        <img src="https://img.shields.io/pypi/pyversions/texrrow.svg" alt="Python Versions" />
+    </a>
+    <a href="https://travis-ci.org/NyanKiyoshi/texrrow/">
+        <img src="https://travis-ci.org/NyanKiyoshi/texrrow.svg?branch=master" alt="Build Status" />
+    </a>
+    <a href="https://codecov.io/gh/NyanKiyoshi/texrrow/">
+        <img src="https://codecov.io/gh/NyanKiyoshi/texrrow/branch/master/graph/badge.svg" alt="Code Coverage" />
+    </a>
+</p>
+
 A simple python web-server to remotely control a LaTeX presentation from a mobile phone to give dynamic and powerful speeches.
 
 ## Development Usage

requirements.txt

@@ -15,3 +15,4 @@ flask-babel
 flask-bootstrap-4-alpha
 flask-wtf
 keyboard
+six

texrrow/core/utils/environ.py

@@ -1,3 +1,4 @@
+from six import raise_from
 from os import environ
 
 E_INVALID_BOOL = '%r is an invalid boolean value.'
@@ -7,7 +8,9 @@ def getenv_or_raise(name, error_message):
     try:
         return environ[name]
     except KeyError as exc:
-        raise KeyError(error_message) from exc
+        exc_value = KeyError(error_message)
+        raise_from(KeyError(error_message), exc)
+        raise exc_value
 
 
 def getenv_int(name, default=None):