* DOC: skip f2py fortran and arrays.nditer cython tests for refguide_check
* DOC: fix formatting of doc tests and improper use of nditer

* Updated f2py "Getting Started" doc to python3.

* WIP: updating f2py docs to python3: intro to python-usage done.

* WIP: updated "Scalar Arguments" session of python-usage.rst

* WIP: updated "String arguments" section of python-usage.rst. TODO check for string bug here.

* WIP: updated "Array arguments" section of python-usage.rst

* WIP: updated "Callback arguments" section in python-usage.rst

* WIP: updated sections "Common blocks" and "F90 module data" from python-usage.rst

* Finished update of python-usage.rst

* WIP: updating usage.rst

* PEP8 fix for equal sign and added note about building extension modules.

* Finished update of usage.rst

* Remove future imports from f2py example.

* Fixed typos.

* Fixed typo.

* Updated f2py "Getting Started" doc to python3.

* Finished update of python-usage.rst

* Finished update of usage.rst

* Fixed typos.

* Addressing comments on PR.

* Addressing PR review; closes gh-14812; fixes gh-14919; closes gh-14960; fixes gh-14865; fixes gh-14862

* Restore names of common block items.

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

Previously, f2py loaded the type mappings from a file ``.f2py_f2cmap``
in current directory, at import time.

Make the file name customizable by adding a ``--f2cmap`` command line
option, and postpone loading the file to f2py.run_main().

Moreover, restore the default type mapping in f2py.run_main() before
loading the customizations, so that multiple calls to f2py.run_main() do
not interfere with each other. (For example, numpy.distutils calls f2py
multiple times in the same process.)