Escape backslashes when serializing marker values

[sueun-dev]

Marker and requirement values are read back with ast.literal_eval (in _parser.process_python_str), which decodes backslash escapes. Value.serialize re-wraps the decoded value in quotes but doesn't re-escape it, so a value containing a backslash doesn't survive a str() round trip:

>>> from packaging.markers import Marker
>>> m = Marker(r'os_name == "C:\\temp"')   # value is C:\temp
>>> str(m)
'os_name == "C:\\temp"'                     # single, unescaped backslash
>>> Marker(str(m)) == m
False                                        # reparses \t as a tab -> C:<TAB>emp

Same for Requirement:

>>> from packaging.requirements import Requirement
>>> r = Requirement(r'demo; os_name == "C:\\temp"')
>>> Requirement(str(r)) == r
False

serialize is the inverse of the ast.literal_eval parser, so it has to escape everything the parser decodes. This is the backslash counterpart of the quote round trip fixed in #1213; the fix doubles backslashes before quoting and leaves the quote-delimiter selection alone.

Added regression tests for Value.serialize and for the Marker/Requirement round trip. Both fail before this change and pass after.

[henryiii]

Suggested change

	# ``process_python_str`` reads a value back with ``ast.literal_eval``,
	# which decodes backslash escapes. Double the backslashes so a value
	# that contains one survives the round trip instead of being re-decoded
	# (e.g. ``C:\temp`` would otherwise reparse to ``C:<TAB>emp``).
	# ``process_python_str`` reads a value with ``ast.literal_eval``,
	# which decodes backslash escapes.

But are there any string-valued markers that might be a path, though? os_name won't be a path. I'm not sure what chars are valid in a node name, that's the main arbitrary one I can think of. I'm not sure the value if there are no markers that can possibly have a backslash.