Remove legacy imports from IPython integration since the "py3compat" module changed in IPython 7.11. (GH-3297)

Make an import prefer Py3 over Py2, since that's what most people are (or should be) using these days.

Import "Iterable" ABC class from "collections.abc" in Py3 instead of deprecated "collections" package.

Closes GH-3246.

Instead of overriding "build_ext.build_extensions()" in "old_build_ext", which works differently for parallel and sequential builds, override only "check_extensions_list()", which gets called by it at the beginning.

Import Cython in "build_ext" only if it's actually needed, i.e. if any Cython implemented extension modules were found.

Simplify the utility code loading by requiring the source file to be named explicitly. It was almost always passed anyway, so having a non-trivial search algorithm in place for a rare case of unnecessary laziness is just code bloat. (GH-3280)

Fix typos

This reverts commit bfcaf163.

This should complete #3171 in silencing -Wmissing-field-initializers.

* IPython magic: hide internal traceback when compilation fails
* IPython magic: hide internal traceback when build fails

This should complete #3171 in silencing -Wmissing-field-initializers.

* IPython magic: hide internal traceback when compilation fails
* IPython magic: hide internal traceback when build fails

Also fix tests in libcpp_algo (how did this ever work?)

Closes GH-1462.