Fix is_color_like for 2-tuple of strings and fix to_rgba for (nth_color, alpha) - #28436

In [6]: to_rgba(('C3', 0.5))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[6], line 1
----> 1 to_rgba(('C3', 0.5))

File ~/git_stuff/matplotlib/lib/matplotlib/colors.py:309, in to_rgba(c, alpha)
    307     rgba = None
    308 if rgba is None:  # Suppress exception chaining of cache lookup failure.
--> 309     rgba = _to_rgba_no_colorcycle(c, alpha)
    310     try:
    311         _colors_full_map.cache[c, alpha] = rgba

File ~/git_stuff/matplotlib/lib/matplotlib/colors.py:391, in _to_rgba_no_colorcycle(c, alpha)
    387             raise ValueError(
    388                 f"Invalid string grayscale value {orig_c!r}. "
    389                 f"Value must be within 0-1 range")
    390         return c, c, c, alpha if alpha is not None else 1.
--> 391     raise ValueError(f"Invalid RGBA argument: {orig_c!r}")
    392 # turn 2-D array into 1-D array
    393 if isinstance(c, np.ndarray):

ValueError: Invalid RGBA argument: 'C3'