docs: Add Pure Python mode to "Calling C functions" tutorial (GH-4247)

docs/examples/tutorial/external/atoi.py

+from cython.cimports.libc.stdlib import atoi
+
+@cython.cfunc
+def parse_charptr_to_py_int(s: cython.p_char):
+    assert s is not cython.NULL, "byte string value is NULL"
+    return atoi(s)  # note: atoi() has no error detection!

docs/examples/tutorial/external/atoi.pyx

 from libc.stdlib cimport atoi
 
+
 cdef parse_charptr_to_py_int(char* s):
     assert s is not NULL, "byte string value is NULL"
     return atoi(s)  # note: atoi() has no error detection!

docs/examples/tutorial/external/keyword_args_call.py

+import cython
+from cython.cimports.strstr import strstr
+
+def main():
+    data: p_char = "hfvcakdfagbcffvschvxcdfgccbcfhvgcsnfxjh"
+
+    pos: p_char = strstr(needle='akd', haystack=data)
+    print(pos is not cython.NULL)

docs/examples/tutorial/external/keyword_args_call.pyx

 cdef extern from "string.h":
     char* strstr(const char *haystack, const char *needle)
 
+
 cdef char* data = "hfvcakdfagbcffvschvxcdfgccbcfhvgcsnfxjh"
 
 cdef char* pos = strstr(needle='akd', haystack=data)

docs/examples/tutorial/external/libc_sin.py

+from cython.cimports.libc.math import sin
+
+@cython.cfunc
+def f(x: cython.double) -> cython.double:
+    return sin(x * x)

docs/examples/tutorial/external/libc_sin.pyx

 from libc.math cimport sin
 
+
 cdef double f(double x):
     return sin(x * x)

docs/examples/tutorial/external/py_version_hex.py

+from cython.cimports.cpython.version import PY_VERSION_HEX
+
+# Python version >= 3.2 final ?
+print(PY_VERSION_HEX >= 0x030200F0)

docs/examples/tutorial/external/strstr.pxd

+cdef extern from "string.h":
+    char* strstr(const char *haystack, const char *needle)

docs/src/tutorial/external.rst

 Calling C functions
 ====================
 
+.. include::
+    ../two-syntax-variants-used
+
 This tutorial describes shortly what you need to know in order to call
 C library functions from Cython code.  For a longer and more
 comprehensive tutorial about using external C libraries, wrapping them
@@ -15,7 +18,17 @@ For example, let's say you need a low-level way to parse a number from
 a ``char*`` value.  You could use the ``atoi()`` function, as defined
 by the ``stdlib.h`` header file.  This can be done as follows:
 
-.. literalinclude:: ../../examples/tutorial/external/atoi.pyx
+.. tabs::
+
+    .. group-tab:: Pure Python
+
+        .. literalinclude:: ../../examples/tutorial/external/atoi.py
+            :caption: atoi.py
+
+    .. group-tab:: Cython
+
+        .. literalinclude:: ../../examples/tutorial/external/atoi.pyx
+            :caption: atoi.pyx
 
 You can find a complete list of these standard cimport files in
 Cython's source package
@@ -28,14 +41,33 @@ Cython also has a complete set of declarations for CPython's C-API.
 For example, to test at C compilation time which CPython version
 your code is being compiled with, you can do this:
 
-.. literalinclude:: ../../examples/tutorial/external/py_version_hex.pyx
+.. tabs::
+
+    .. group-tab:: Pure Python
+
+        .. literalinclude:: ../../examples/tutorial/external/py_version_hex.py
+            :caption: py_version_hex.py
+
+    .. group-tab:: Cython
+
+        .. literalinclude:: ../../examples/tutorial/external/py_version_hex.pyx
+            :caption: py_version_hex.pyx
 
 .. _libc.math:
 
 Cython also provides declarations for the C math library:
 
-.. literalinclude:: ../../examples/tutorial/external/libc_sin.pyx
+.. tabs::
 
+    .. group-tab:: Pure Python
+
+        .. literalinclude:: ../../examples/tutorial/external/libc_sin.py
+            :caption: libc_sin.py
+
+    .. group-tab:: Cython
+
+        .. literalinclude:: ../../examples/tutorial/external/libc_sin.pyx
+            :caption: libc_sin.pyx
 
 Dynamic linking
 ---------------
@@ -83,6 +115,9 @@ This allows the C declaration to be reused in other Cython modules,
 while still providing an automatically generated Python wrapper in
 this specific module.
 
+.. note:: External declarations must be placed in a ``.pxd`` file in Pure
+    Python mode.
+
 
 Naming parameters
 -----------------
@@ -103,7 +138,19 @@ You can now make it clear which of the two arguments does what in
 your call, thus avoiding any ambiguities and often making your code
 more readable:
 
-.. literalinclude:: ../../examples/tutorial/external/keyword_args_call.pyx
+.. tabs::
+
+    .. group-tab:: Pure Python
+
+        .. literalinclude:: ../../examples/tutorial/external/keyword_args_call.py
+            :caption: keyword_args_call.py
+        .. literalinclude:: ../../examples/tutorial/external/strstr.pxd
+            :caption: strstr.pxd
+
+    .. group-tab:: Cython
+
+        .. literalinclude:: ../../examples/tutorial/external/keyword_args_call.pyx
+            :caption: keyword_args_call.pyx
 
 Note that changing existing parameter names later is a backwards
 incompatible API modification, just as for Python code.  Thus, if