Fix version comparison

Author: pllim

pytest_mpl/plugin.py

@@ -57,7 +57,8 @@
   Actual shape: {actual_shape}
     {actual_path}"""
 
-PYTEST_LT_7 = Version(pytest.__version__) < Version("7.0.0")
+_pytest_version = Version(pytest.__version__)
+PYTEST_LT_7 = _pytest_version < Version("7.0.0")
 PYTEST_GE_8_0 = any([_pytest_version.is_devrelease,
                      _pytest_version.is_prerelease,
                      _pytest_version >= Version('8.0')])