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

DOC: Fixed name of .so file in userguide's numpy_tutorial.

"convolve_cy.so" -> "compute_cy.so" to match imports used in this tutorial.

Mark optimized abs as being nogil-safe.

Prevent dynamically multiplied tuple constants from being deduplicated. Only the constant part is considered for them.

Prevent repeated initialisation of constants that overwrite the previously created objects without cleanup.
Closes #2750.

Fix sigismember() declaration

Don't try to use imap with python3

Still requires the more conservative __index__ here rather than a possibly
truncating __int__ because this is used in a context where floating point
values should probably be treated specially.

This fixes Github issue #2752.

Reduce warning level in MSVC again since /W4 turns out to be excessive (complains about constant conditionals, for example).

Closes #2736.

Increase warning level in appveyor builds (could also use "/Wall", but let's see how spammy /W4 is).

Closes #2731.