[gobject-introspection] tests: Fix compatibility with Python 3.5

[Christoph Reiter <creiter src gnome org>]

commit c0beff705c4c67f6ffe33abbe52a8dd9a30903ab
Author: Tomasz Miąsko <tomasz miasko gmail com>
Date:   Fri Mar 15 00:00:00 2019 +0000

    tests: Fix compatibility with Python 3.5
    
    Use call_count to verify the number of calls. This replaces
    assert_called_once used previously, which is only available
    since Python 3.6.
    
    Fixes issue #274.

 tests/scanner/test_ccompiler.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/tests/scanner/test_ccompiler.py b/tests/scanner/test_ccompiler.py
index dd8700d8..5890aa9b 100644
--- a/tests/scanner/test_ccompiler.py
+++ b/tests/scanner/test_ccompiler.py
@@ -85,7 +85,7 @@ class UnixCCompilerTest(unittest.TestCase):
                 # Don't actually do anything.
                 cc.compiler.dry_run = True
                 cc.compile(pkg_config_cflags, cpp_includes, [source], init_sections)
-        spawn.assert_called_once()
+        self.assertEqual(1, spawn.call_count)
         args, kwargs = spawn.call_args
         return args[0]
 
@@ -106,7 +106,7 @@ class UnixCCompilerTest(unittest.TestCase):
                 # Don't actually do anything.
                 cc.compiler.dry_run = True
                 cc.preprocess(source, output, cpp_options)
-        spawn.assert_called_once()
+        self.assertEqual(1, spawn.call_count)
         args, kwargs = spawn.call_args
         return args[0]