packaging: leverage os.path.relpath() in setup.py
`os.path.relpath()` has existed since Python 2.6, so we can safely use
it. This fixes a bug in the current code when the common prefix is "/"
(in which case `uplevel` would be one less than it should).
Differential Revision: https://phab.mercurial-scm.org/D7875
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false