pep8
- pep8
 - removed mutable default values
 - foo == None -> foo is None
Move external libraries to 'extern' directory
Merge pull request #2531 from ianthomas23/extern_directory

Move external libraries to 'extern' directory
cast argv to unicode before testing

otherwise the use of `unicode-literals` causes errors
when there are non-ascii characters on argv on Python 2.
Merge pull request #2534 from minrk/unicode-argv

cast argv to unicode before testing
Fix for compiling ttconv in extern directory
Merge pull request #2535 from ianthomas23/extern_directory_fix

Move external libraries to 'extern' directory - correction
add more spectrum functions and add new spectrum types to specgram in mlab.py, and clean up window and detrend functions in mlab.py
add mlab tests for new spectrum types, window and detrend functions, and greatly expand existing tests
add CleanupTestCase, a unittest.TestCase with cleanup
refactor mlab spectrum, window, and detrend functions to improve modularity, testability, and performance
implement tests for refactored mlab spectrum, windowing, and detrend functions, with 100% coverage
add more spectral plots and add new spectrum types to specgram in _axes
add tests for new spectrum types in _axes
add demo for new spectral plot types
added information regarding new spectrum functions to CHANGELOG, whats_new, and api_changes
Merge pull request #2522 from toddrjen/spectrum

Add additional spectrum-related plots and improve underlying structure
Merge pull request #2514 from tacaswell/mpl_toolkit_pep8

Mpl toolkit pep8
fix removed api change regarding spectral functions