* Implement `SpecCompose` for applying a pipeline of spectrogram tran…

…sforms. Thanks to omerferhatt.

* Fix a bug in `SpecChannelShuffle` where it did not support more than 3 audio channels. Thanks to omerferhatt.
* Limit scipy version range to >=1.0,<1.6 to avoid issues with loading 24-bit wav files.
Support for scipy>=1.6 will be added later.

* Fix picklability of instances of `AddImpulseResponse`, `AddBackgrou…

…ndNoise`

 and `AddShortNoises`
* Add an option `leave_length_unchanged` to `AddImpulseResponse`

* Implement `LoudnessNormalization`

* Implement `randomize_parameters` in `Compose`. Thanks to SolomidHero.
* Add multichannel support to `AddGaussianNoise`, `AddGaussianSNR`, `ClippingDistortion`, `FrequencyMask`, `PitchShift`, `Shift`, `TimeMask` and `TimeStretch`

* Show a warning if a waveform had to be resampled after loading it. …

…This is because resampling

is slow. Ideally, files on disk should already have the desired sample rate.
* Correctly find audio files with upper case filename extensions.
* Lay the foundation for spectrogram transforms. Implement `SpecChannelShuffle` and
`SpecFrequencyMask`.
* Fix a bug where AddBackgroundNoise crashed when trying to add digital silence to an input. Thanks to juheeuu.
* Configurable LRU cache for transforms that use external sound files. Thanks to alumae.
* Officially add multichannel support to `Normalize`

* Speed up AddBackgroundNoise, AddShortNoises and AddImpulseResponse …

…by loading wav files with scipy or wavio instead of librosa.

* Implement `Mp3Compression`

* Python <= 3.5 is no longer officially supported, since [Python 3.5 has reached end-of-life](https://devguide.python.org/#status-of-python-branches)
* Expand range of supported `librosa` versions
* Officially support multichannel audio in `Gain` and `PolarityInversion`
* Add m4a and opus to the list of recognized audio filename extensions
* Breaking change: Internal util functions are no longer exposed directly. If you were doing
    e.g. `from audiomentations import calculate_rms`, now you have to do
    `from audiomentations.core.utils import calculate_rms`

* Implement `Gain` and `PolarityInversion`. Thanks to Spijkervet for …

…the inspiration.

* Breaking change: AddImpulseResponse, AddBackgroundNoise and AddShor…

…tNoises now include sounds from subfolders. This is useful when your sound files are organized in subfolders.

* AddImpulseResponse, AddBackgroundNoise and AddShortNoises now support aiff files in addition to flac, mp3, ogg and wav
* Fix filter instability bug in FrequencyMask. Thanks to kvilouras.

* Disregard non-audio files when looking for impulse response files

* Remember randomized/chosen effect parameters. This allows for freezing the parameters and applying the same effect to multiple sounds. Use transform.freeze_parameters() and transform.unfreeze_parameters() for this.
* Fix a bug in ClippingDistortion where the min_percentile_threshold was not respected as expected.
* Implement transform.serialize_parameters(). Useful for when you want to store metadata on how a sound was perturbed.
* Switch to a faster convolve implementation. This makes AddImpulseResponse significantly faster.
* Add a rollover parameter to Shift. This allows for introducing silence instead of a wrapped part of the sound.
* Expand supported range of librosa versions
* Add support for flac in AddImpulseResponse
* Implement AddBackgroundNoise transform. Useful for when you want to add background noise to all of your sound. You need to give it a folder of background noises to choose from.
* Implement AddShortNoises. Useful for when you want to add (bursts of) short noise sounds to your input audio.
* Improve handling of empty input

* Add shuffle parameter in Composer

* Add Resample transformation
* Add ClippingDistortion transformation
* Add SmoothFadeTimeMask as alternative to TimeMask

Thanks to askskro