Add new "language_level=3str"

 ENH: add check_size option to ctypedef class for external classes 

Remove the new 'str_is_str' directive again and replace it by a new "language_level=3str" that resembles "language_level=3" but keeps unprefixed string literals and the 'str' builtin type unchanged.
See #2565.

Closes #2632.

Ignore the "c_string_encoding" directive when looking for a "coding" cookie in the first two source lines.
Closes #2632.

DOC, TST: test and document external extension type attribute aliasing

MAINT: if subtree test fails, print out all stdout, stderr not just last

Add a new directive 'str_is_str=True'

Add a new directive 'str_is_str=True' that keeps unprefixed string literals and the 'str' builtin type unchanged even when 'language_level=3' is enabled.
See #2565.

TEST: add test for 'binary incompatibility' warning

Deduplicate constant tuples and slices to generate them only once and share them across the entire module.
Closes #2292.

Previously, line tracing code could be generated before the case-statements and thus be ignored at best or rejected by the C compiler.

Use a different constant (PI instead of NAN) in the "import *" test since NAN does not seem to be defined by MSVC.

Call out the different name used for the wrapper

Allow catching both C++ and Python exceptions.

Explicitly call out the different name used for the wrapper `.pyx` file vs. the other C++ sources

As a new user, I was following along and just called the `.pyx` file `Rectangle.pyx` like everything else which was kinda confusing then when in the `wrapping_CPlusPlus/setup.py` there's a `setup(ext_modules=cythonize("rect.pyx"))` and I was thinking that maybe that file was magically created somehow and was miffed.

Hoisted the blurb on why the file name needs to be different without some additional work and worded it in there.