GCC diagnostic pragmas were introduced in GCC 4.6, so augment the GCC
compiler check with a version check to avoid compilation errors with GCC
< 4.6.

REF: make Utils dependency-free by moving search_include_directories

REF: Move CompilationOptions and default_options to Options

update imports
remove Options.create_context in favor of Context.from_options

Avoid "__name__" hack in "Method" scanner action class and replace it with a simple "__repr__()" implementation.

Make the "Method" scanner action a direct feature of the embedded Plex, instead of hacking it back into the Lexicon with a stray import.
See GH-2831.

Resolve flake8 findings in Cython.Utils.

Closes #2802.

Remove unused code, mostly from Plex.

Misc. typo fixes

Found via `codespell -q 3 -L uint,sinc,nd,byteorder,delimeter,delimeters,te,iff,splitted,highlightning,tripple,varius,includee,ist,ue,ba,defin,amin,ith,creat,minimise,crasher`

Add safety cast in queue tutorial code that avoids casting a potentially non-pointer-sized "int" to a "void*".

Silence an "unused" C compiler warning when we cannot easily decide whether to include the "__Pyx_Method_ClassMethod" utility code or not.

Add safety parentheses in helper macro, also to avoid a compiler warning about tautological comparisons.

implement setup.cfg, lint cleanup of Plex directory

Make the cpdef override check work with Python subclasses of extension types again that do not initialise their own dict.
This lead to a crash with dict versioning in Py3.6+.
Closes GH-2823.

Do not install test dependencies on CPython dev versions in travis as they are likely to fail in pre-releases.

Tighten grammar to disallow a mix of Py2 style octal literals with Py3 style underscores: "07" is still allowed, "0_7" is not and must be spelled "0o_7" and the like.
Basically, you get either Py2 style legacy octals or Py3 style underscore literals, but you have to decide what you want.