PyPy 3 fixes

Note that on pypy, SOABI tags start with '.pypy' which contains
the string '.py'...

It is used even when CYTHON_USE_TYPE_SLOTS is disabled.

Disable it on pypy3, since it doesn't support PEP 442 yet.

Remove special case support for block name suffix after "XYZ.proto" utility code title. Replaced by existing "@proto_block" option.

Refactor coercion predicate function to make it clearer what happens for the template based conversions and for the non-template string conversions.

Avoid deprecation warning for "Py_UNICODE_strlen()" by always using the version that we ship anyway.

Improve C macro guard: __Pyx_GetAttr() should be optimised exactly iff __Pyx_PyObject_GetAttrStr() is also optimised.

Avoid inlining some generic fallback functions. However short they are, they are just fallbacks and shouldn't get in the way.

Refactor dependencies between buffer/memoryview utility code to avoid unused C code and remove explicit utility code dependency lists from Python code where possible since they are much better expressed right in the C code.

Split 2-tuple unpacking helper function into a (still) inlineable fast-path and a slow fallback function that does not need inlining.

Also avoid double-check overhead for strigns and string subtypes in Py2: only needed in Py2.6 and not really worth it.

Supply a general macro for fast exception setting without value (e.g. StopIteration).

Mark intended fall-through steps in switch statement of int-pow() implementation with "CYTHON_FALLTHROUGH" to avoid compiler warnings.

Avoid excessive inlining of __Pyx_GetBufferAndValidate() helper and only inline the initial None check through a macro.
Assumption is that explicit None assignments to buffer variables are somewhat common to clear data and NULL checks can be eliminated, but everything else takes substantial time anyway.