Closes #2820.

See #2807.

Closes #1982.

It fixes error 
```
src\lxml\etree.c(241210) : error C2275: 'PyObject' : illegal use of this type as an expression
```
when installing lxml 4.3.0 to pypy2 v6.0.0 on Windows.

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