Add pthread tag to cypclass tests

tests/errors/cypclass_constructions_error.pyx

 # mode: error
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass UnknownNewOptionalArgs:

tests/errors/cypclass_gil_error.pyx

 # mode: error
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 def GILHoldingFunction():

tests/errors/cypclass_self_error.pyx

 # mode: error
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass MethodsWithoutSelf:

tests/run/cypclass_constructor_type_inference.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass SomeMemory:

tests/run/cypclass_multiple_inheritance.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass Base:

tests/run/cypclass_new_alloc.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 

tests/run/cypclass_new_statement.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass Nothing:

tests/run/cypclass_templates.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass TemplatedBase[T, U]:

tests/run/cypclass_wrapped_constructor.pyx

 # mode: run
-# tag: cpp, cpp11
+# tag: cpp, cpp11, pthread
 # cython: experimental_cpp_class_def=True, language_level=2
 
 cdef cypclass Default: