Allow alternative space characters as group separator when parsing numbers (#1007)

Allow alternative space characters as group separator when parsing numbers (#1007)

The French group separator is `"\u202f"` (narrow non-breaking space),
but when parsing numbers in the real world, you will most often encounter
either a regular space character (`" "`) or a non-breaking space character
(`"\xa0"`).

The issue was partially adressed earlier in https://github.com/python-babel/babel/issues/637,
but only to allow regular spaces instead of non-breaking spaces `"\xa0"` in
`parse_decimal`.

This commit goes further by changing both `parse_number` and `parse_decimal`
to allow certain other space characters when the group character is itself a space character,
but is not present in the string to parse.

Unit tests are included.