Import template functions from Home Assistant into pyscript

Home Assistant template functions in pyscript

Inspired by [pyscript#251](https://github.com/custom-components/pyscript/issues/251) and [pyscript#568](https://github.com/custom-components/pyscript/issues/568).

This is a module that you can put in your [pyscript](https://github.com/custom-components/pyscript) modules directory and then import from any pyscript script and have access to all the template functions that Home Assistant provides.

Features:
* It automatically determines what functions Home Assistant is providing to templates, so when new functions get added by a Home Assistant update (like labels and areas did recently) they get included automatically.
* It even includes things added by extensions, like the template functions added by [spook](https://spook.boo/).
* It automatically figures out which functions need the `hass` instance passed to them and wraps the ones that do.

## Usage ##

```python
import template_functions as tf

for area in tf.areas():
  print(tf.area_name(area))
```

template_functions.py

"""Import template functions from Home Assistant into pyscript."""

import functools
import inspect
import builtins
import homeassistant.helpers.template

def get_hass_globals():
    """Get all the global functions that are defined in Home Assistant templates"""
    _env = homeassistant.helpers.template.TemplateEnvironment(hass)
    res = {}

    def needs_hass_wrapper(func):
        """Determine if a value is a function that needs to be bound to the hass object"""
        if not inspect.isfunction(func):
            return False
        sig = inspect.signature(func)
        for p in sig.parameters.values():
            if p.name == "hass":
                return True
        return False

    # Make a list of builtins so we can skip installing any template functions
    # that shadow builtins (mainly `list` and `dict`)
    _builtins = dir(builtins)

    # Get all the globals, and any filters that are not also globals
    for src in (_env.globals, _env.filters):
        for key, value in src.items():
            if key in _builtins or key in res:
                continue
            if needs_hass_wrapper(value):
                res[key] = functools.partial(value, hass)
            else:
                res[key] = value
    return res

globals().update(get_hass_globals())