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.

* Rearrange libcpp.algorithm to match cppreference and reformat
* Add all_of, any_of and none_of to libcpp.algorithm
* Add count and count_if to libcpp.algorithm
* Add string(first, last) constructor to libcpp.string
* Add the simplest overload of mismatch to libcpp.algorithm
* Add find, find_if, and find_if_not to libcpp.algorithm
* Add find_end to libcpp.algorithm and distance to libcpp.iterator
* Add find_first_of to libcpp.algorithm
* Add adjacent_find to libcpp.algorithm
* Add search to libcpp.algorithm
* Add search_n to libcpp.algorithm
* Add exception specifiers in libcpp.algorithm
* Add predicated overload of find_end, search and search_n to libcpp.algorithm

This is more follow up to https://bugs.python.org/issue37250

The action taken is:
 - restore tp_print to not break all of the sdists on pypi for py38
 - remove tp_print for real in py39

In https://github.com/cython/cython/pull/3171 tp_print was initialized
for PY_VERSION_HEX >= 0x030800b4 however, when trying to use cython
with cpython master (aka py39) there are compile time exceptions due
to too many initializers:

   error: too many initializers for ‘PyTypeObject’ {aka ‘_typeobject’}

This fixes that by putting an upper bound on the ifdef of including
pt_print at the end of the object.