Change File and Directory trait descriptions to take into account the 'exists' flag.

[mdickinson]

This PR customises the trait description for the File and Directory trait types to take into account whether exists was true or not.

For the File trait, the description has changed from "a filename or object implementing the os.PathLike interface" to "a string or os.PathLike object" in the normal case and "a string or os.PathLike object referring to an existing file" for the File(exists=True) case. The changes for Directory are analogous.

Before:

>>> from traits.api import Directory, File, HasStrictTraits
>>> class A(HasStrictTraits):
...     home_dir = Directory(exists=True)
...     config_file = File(exists=True)
... 
>>> a = A()
>>> a.home_dir = 47
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 1524, in validate
    validated_value = super().validate(
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 350, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 74, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'home_dir' trait of an A instance must be a directory name or an object implementing the os.PathLike interface, but a value of 47 <class 'int'> was specified.
>>> a.config_file = 47
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 1395, in validate
    validated_value = super().validate(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 350, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 74, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'config_file' trait of an A instance must be a filename or object implementing the os.PathLike interface, but a value of 47 <class 'int'> was specified.

After:

>>> from traits.api import Directory, File, HasStrictTraits
>>> class A(HasStrictTraits):
...     home_dir = Directory(exists=True)
...     config_file = File(exists=True)
... 
>>> a = A()
>>> a.home_dir = 47
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 1524, in validate
    validated_value = super().validate(
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 350, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 74, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'home_dir' trait of an A instance must be a string or os.PathLike object referring to an existing directory, but a value of 47 <class 'int'> was specified.
>>> a.config_file = 47
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 1392, in validate
    validated_value = super().validate(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 350, in validate
    self.error(object, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/base_trait_handler.py", line 74, in error
    raise TraitError(
traits.trait_errors.TraitError: The 'config_file' trait of an A instance must be a string or os.PathLike object referring to an existing file, but a value of 47 <class 'int'> was specified.

Fixes #1439

@nicolasap-dm: Do you have bandwidth for a quick review?

Checklist

• Tests
• [ ] Update API reference (docs/source/traits_api_reference) N/A
• [ ] Update User manual (docs/source/traits_user_manual) N/A
• [ ] Update type annotation hints in traits-stubs N/A

[nicolasap-dm]

LGTM, thanks