Use black formatting, update code style and conventions, miscellaneous pytest fixes

This commit contains necessary "Test"-prefix renamings.  Some test classes
were being automatically generated and `pytest` wouldn't pick them up unless
they matched the correct pattern.  Plus, the naming was all over the place, so
such changes were warranted anyway.

There were also some `pytest` constructor issues causing test classes to be
ignored.  In those cases, `setup_method` was used instead of `__init__`.

Closes #5.

Move from unittest/nose to pytest

Closes #6685.

* Fix subtensor numpy warning

* Fix simple typo: varible->size -> variable->size

Closes #6734

* Counteract hypot redefinition for old Python versions

* fix doc warning

* Update extending_theano.txt

* OrderedDict cannot be imported from collections.abc, it is not abstract

See https://github.com/Theano/Theano/pull/6664#issuecomment-581845517

* Move more collections.abc imports to theano.compat

* Use math.gcd instead of fractions.gcd when possible

fractions.gcd() function has been removed from Python 3.9, it was
deprecated since Python 3.5.

https://docs.python.org/3.9/whatsnew/3.9.html#removed

* DOC: Allow building with Sphinx >= 2.0

* MAINT: Avoid SyntaxWarnings on import in Python 3.8

* flake8 code style fixes

* DOC: Retain compatibility with older Sphinx

* MAINT: Be compatible with numpy 1.17 and scipy 1.3

Fixes #6715.  Includes and supersedes #6721.

* flake8 code style fixes

* Remove doctest in python 3.4

* Do not install sphinx_rtd_theme for python 3.4 as the installation doesn't work anymore.

* fixing typo in GammaIncC Op

* re-add min > max clip test with fixed reference

* DOC: min > max clip may not match numpy

* updating travis build for Python 3

* order of stages

* conda activate

* installing conda properly

* conda install on travis

* conda activate issues on travis

* conda activate issues on travis

* doctest needs older version of numpy

* doctest

* reverting setup.cfg

* conda version conflicts

* need old sphinx version

* sphinx version

* don't bother building docs

* moved nosetester import

* also moving known failures plugin
Co-authored-by: Adrian Seyboldt <adrian.seyboldt@gmail.com>
Co-authored-by: Frédéric Bastien <frederic.bastien@gmail.com>
Co-authored-by: Tim Gates <tim.gates@iress.com>
Co-authored-by: Marcel Bargull <marcel.bargull@udo.edu>
Co-authored-by: Arnaud Bergeron <abergeron@gmail.com>
Co-authored-by: Miro Hrončok <miro@hroncok.cz>
Co-authored-by: Rebecca N. Palmer <rebecca_palmer@zoho.com>
Co-authored-by: Frederic Bastien <fbastien@nvidia.com>

Adding support for building docs on ReadTheDocs

Update to head of upstream Theano master branch

Adding support for building docs on ReadTheDocs

Adding support for building docs on ReadTheDocs

rtds requirements

trying conda env on rtds

typo in env file

trying to install recent sphinx

installing rtds deps

installing rtds deps

channel priority

reverting channel priority

cleaning up

MAINT: Be compatible with numpy 1.17 and scipy 1.3

Fixing typo in GammaIncC Op

Use conda-forge for Python 3.4 to fix travis

DOC: Allow building with Sphinx >= 2.0

Fixes #6715.  Includes and supersedes #6721.