Accessing the first element of a `std::vector` that is empty results in
a crash.

The __PYX_XDEC_MEMVIEW statements generated for try/finally statements
varied per build if there were multiple types of variables to be cleaned
up.

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

This fixes the following compiler warning:
  warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]

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.

Closes #2736.

Closes #2731.

Closes #2671.

Closes #2729.

Plus Start filling unit test cases

“It have”-> “it has”
“Parralelism” -> “parallelism”

Also add a compile time check for SIZEOF_VOID_P.

This fixes #2670.

See also
https://bugs.python.org/issue35037
https://bugs.python.org/issue4709