[BUG] Calling 'operator bool' results in 'AttributeError'

[lkollar]

Describe the bug
Declaring a C++ string and then invoking an if <identifier>: operation on it raises an AttributeError:

[1/1] Cythonizing test.pyx
Traceback (most recent call last):
  File "/Users/lkisskollar/tmp/cython_bug/setup.py", line 4, in <module>
    setup(name="test", version="0.1.0", ext_modules=cythonize(["test.pyx"], language_level=3))
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Build/Dependencies.py", line 1116, in cythonize
    cythonize_one(*args)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Build/Dependencies.py", line 1262, in cythonize_one
    result = compile_single(pyx_file, options, full_module_name=full_module_name)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Main.py", line 577, in compile_single
    return run_pipeline(source, options, full_module_name)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Main.py", line 505, in run_pipeline
    err, enddata = Pipeline.run_pipeline(pipeline, source)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Pipeline.py", line 372, in run_pipeline
    data = run(phase, data)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Pipeline.py", line 352, in run
    return phase(data)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Pipeline.py", line 52, in generate_pyx_code_stage
    module_node.process_implementation(options, result)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/ModuleNode.py", line 206, in process_implementation
    self.generate_c_code(env, options, result)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/ModuleNode.py", line 501, in generate_c_code
    self.body.generate_function_definitions(env, code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 403, in generate_function_definitions
    stat.generate_function_definitions(env, code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 2067, in generate_function_definitions
    self.generate_function_body(env, code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 1827, in generate_function_body
    self.body.generate_execution_code(code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 409, in generate_execution_code
    stat.generate_execution_code(code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 6798, in generate_execution_code
    if_clause.generate_execution_code(code, end_label, is_last=i == last)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Nodes.py", line 6837, in generate_execution_code
    self.condition.generate_evaluation_code(code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/ExprNodes.py", line 6210, in generate_evaluation_code
    self.function.generate_evaluation_code(code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/ExprNodes.py", line 829, in generate_evaluation_code
    self.generate_result_code(code)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/ExprNodes.py", line 7360, in generate_result_code
    code.intern_identifier(self.attribute),
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Code.py", line 1948, in intern_identifier
    return self.get_py_string_const(text, identifier=True)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Code.py", line 1938, in get_py_string_const
    return self.globalstate.get_py_string_const(
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Code.py", line 1379, in get_py_string_const
    c_string = self.get_string_const(text)
  File "/Users/lkisskollar/tmp/cython_bug/venv/lib/python3.9/site-packages/Cython/Compiler/Code.py", line 1350, in get_string_const
    if text.is_unicode:
AttributeError: 'str' object has no attribute 'is_unicode'

To Reproduce
Code to reproduce the behaviour:

# distutils: language=c++

from libcpp.string cimport string

cdef foo():
    cdef string field
    if field:
        pass

Expected behavior
No compiler crash and an error message that this is an invalid operation.

Environment (please complete the following information):

• OS: [e.g. Linux, Windows, macOS] macOS
• Python version [e.g. 3.8.4] 3.9.4
• Cython version [e.g. 0.29.18] 0.29.24 and 3.0.0a9