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.

Only inject eval() locals/globals args when it's called as a builtin and not redefined e.g. in the module (found when updating test_grammar() and overriding it there).

When detecting an invalid system configuration for overflow checks, at least print a warning, instead of leaving a dangling live exception behind.

The current code fails to compile with:

<source>:3:54: error: invalid operands to binary expression ('enum E' and 'enum E')
const enum E neg_one_class_29 = (enum E) ((enum E) 0 - (enum E) 1);
                                          ~~~~~~~~~~ ^ ~~~~~~~~~~

This change reverts to the code that was before #2186 but silences erroneous GCC warning enabled by -Wconversion (which is not a part of -Wall).

Fixes #2749