py-voluptuous: Update to 0.11.7
Upstream changes are many minor improvements and bugfixes, plus
**Changes**
- [#378](https://github.com/alecthomas/voluptuous/pull/378): Allow `extend()` of a `Schema` to return a subclass of a `Schema` as well.
**Changes**:
- [#349](https://github.com/alecthomas/voluptuous/pull/349): Support Python 3.7.
- [#343](https://github.com/alecthomas/voluptuous/pull/343): Drop support for Python 3.3.
**Changes**:
- [#293](https://github.com/alecthomas/voluptuous/pull/293): Support Python 3.6.
- [#294](https://github.com/alecthomas/voluptuous/pull/294): Drop support for Python 2.6, 3.1 and 3.2.
- [#318](https://github.com/alecthomas/voluptuous/pull/318): Allow to use nested schema and allow any validator to be compiled.
- [#324](https://github.com/alecthomas/voluptuous/pull/324):
Default values MUST now pass validation just as any regular value. This is a backward incompatible change if a schema uses default values that don't pass validation against the specified schema.
- [#328](https://github.com/alecthomas/voluptuous/pull/328):
Modify `__lt__` in Marker class to allow comparison with non Marker objects, such as str and int.
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