Use python3 for tests

The testcase python files were already compatible with python3, but the
shebang was wrongly set to just 'python'

tests/testcases/basic.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/clear_shadow_unredirected.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue239.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue239_2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue239_3.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue239_3_norefresh.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue314.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue314_2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue314_3.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue357.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue394.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue465.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/issue525.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib

tests/testcases/redirect_when_unmapped_window_has_shadow.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import xcffib.xproto as xproto
 import xcffib