set language_level=3 for Cython

[grlee77]

We do not currently specify the Cython language_level. This results in a warning from Cython 0.29 that the default is going to change from language_level=2 to language_level=3str in the future.

As we are dropping Python 2 support in the next release, I thought we should just explicitly specify language_level=3. This PR adds the language_level directive at the top of each .pyx file.

There was one place where / needed to be // to maintain integer division. The rest of the changes are to the import statements and were necessary to get the files to compile under this language level.

This is built on top of #434 so that the Python 2.7 test cases wouldn't be run. Only the c7c9c2f commit is new to this PR.

[grlee77]

I rebased this on current master to get rid of all the already merged commits from #434. I then moved the language_level statement to setup.py as suggested by @tacaswell in #451 since it is applied globally.

[grlee77]

The only change here to the Cython code aside from import syntax was the need to specify integer division at the following location:
https://github.com/PyWavelets/pywt/pull/435/files#diff-c8a5633b1197b648cc4d0c0949ed195eL1047

[rgommers]

LGTM, merged. Thanks @grlee77

[grlee77]

I got a notification about failures on ReadtheDocs and will look into it. We probably just need to make sure the Cython version being used there is recent enough.

https://readthedocs.org/projects/pywavelets/builds/8509525/

[rgommers]

Ah yes, we're still pinning to Cython 0.20.2 in util/readthedocs/requirements.txt

[grlee77]

#452 should fix the documentation build