Accept export KEY=value lines in .env files.

docker-compose and python-dotenv both treat a leading export as
optional syntax. We were storing the key as "export KEY".

Author: gyanu2507

decouple.py

@@ -152,6 +152,10 @@ def __init__(self, source, encoding=DEFAULT_ENCODING):
                 line = line.strip()
                 if not line or line.startswith('#') or '=' not in line:
                     continue
+                if line.startswith('export ') or line.startswith('export\t'):
+                    line = line[7:].lstrip()
+                    if not line or '=' not in line:
+                        continue
                 k, v = line.split('=', 1)
                 k = k.strip()
                 v = v.strip()

tests/test_env.py

@@ -49,6 +49,8 @@
 KeyHasTwoDoubleQuote='"Y"'
 KeyHasMixedQuotesAsData1="Y'
 KeyHasMixedQuotesAsData2='Y"
+export ExportedKey=fromexport
+export	ExportedTab=tabs
 '''
 
 @pytest.fixture(scope='module')
@@ -140,3 +142,8 @@ def test_env_with_quote(config):
 def test_env_repo_keyerror(config):
     with pytest.raises(KeyError):
         config.repository['UndefinedKey']
+
+
+def test_env_export_prefix(config):
+    assert 'fromexport' == config('ExportedKey')
+    assert 'tabs' == config('ExportedTab')