feat: Remove obsolete LANGUAGE_LEVEL argument

Starting with Cython 3.0, language level defaults to "3str".

See https://cython.readthedocs.io/en/latest/src/userguide/migrating_to_cy30.html#python-3-syntax-semantics

Co-authored-by: Jean-Christophe Fillion-Robin <[email protected]>

Author: henryiii

src/cython_cmake/cmake/UseCython.cmake

@@ -9,7 +9,7 @@
 #
 #   Cython_compile_pyx(<pyx_file1> [<pyx_file2> ...]
 #                     [LANGUAGE C | CXX]
-#                     [LANGUAGE_LEVEL 2 | 3 | 3str]
+#                     [CYTHON_ARGS <args> ...]
 #                     [OUTPUT_VARIABLE <OutputVariable>])
 #
 # Options:
@@ -19,15 +19,6 @@
 #   generated, unless the C language is not enabled for the project; in this
 #   case, a C++ file is generated by default.
 #
-# ``LANGUAGE_LEVEL [2 | 3 | 3str]``
-#   Force compilation using either Python-2, Python-3 or Python-3str syntax and
-#   code semantics.
-#   By default, Python-3str syntax and semantics are used if the major version
-#   of Python found is 3 and Cython >=3 is used. Otherwise, Python-2 syntax and
-#   semantics are used.
-#   Using the Python-3str syntax and semantics means there cython does not assume
-#   unicode is used by by default for string literals under Python 2
-#
 # ``CYTHON_ARGS <args>``
 #   Specify additional arguments for the cythonization process.
 #
@@ -76,7 +67,7 @@
 
 function(Cython_compile_pyx)
   set(_options )
-  set(_one_value LANGUAGE_LEVEL LANGUAGE OUTPUT_VARIABLE)
+  set(_one_value LANGUAGE OUTPUT_VARIABLE)
   set(_multi_value CYTHON_ARGS)
 
   cmake_parse_arguments(_args
@@ -112,22 +103,6 @@ function(Cython_compile_pyx)
   set(_language_arg ${_language_${_language}_arg})
   set(_language_extension ${_language_${_language}_extension})
 
-  # Set language level
-  set(_language_level ${_args_LANGUAGE_LEVEL})
-  if(NOT _args_LANGUAGE_LEVEL)
-    set(_language_level "3str")
-  else()
-    if(NOT _language_level MATCHES "^(2|3|3str)$")
-      message(FATAL_ERROR "LANGUAGE_LEVEL must be one of 2, 3, or 3str")
-    endif()
-  endif()
-
-  set(_language_level_2_arg "-2")
-  set(_language_level_3_arg "-3")
-  set(_language_level_3str_arg "--3str")
-
-  set(_language_level_arg "${_language_level_${_language_level}_arg}")
-
   # Generated depfile is expected to have the ".dep" extension and be located along
   # side the generated source file.
   set(_depfile ${generated_file}.dep)
@@ -153,7 +128,6 @@ function(Cython_compile_pyx)
       COMMAND ${CYTHON_EXECUTABLE}
       ARGS
         ${_language_arg}
-        ${_language_level_arg}
         ${_args_CYTHON_ARGS}
         ${_depfile_arg}
         ${pyx_location}