Speed up attribute lookups that do not care about the AttributeError

Add a new utility function "__Pyx_PyObject_GetAttrStrNoError()" that does not raise AttributeError if the lookup fails, and use "_PyObject_GenericGetAttrWithDict()" for it in Py3.7+ to avoid instantiating the exception completely.

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).