bpo-20523: pdb searches for .pdbrc in ~ instead of $HOME (GH-11847)

Previously pdb checked the $HOME environmental variable
to find the user .pdbrc. If $HOME is not set, the user
.pdbrc would not be found.

Change pdb to use `os.path.expanduser('~')` to determine
the user's home directory. Thus, if $HOME is not set (as
in tox or on Windows), os.path.expanduser('~') falls
back on other techniques for locating the user's home
directory.

This follows pip's implementation for loading .piprc.

Co-authored-by: Dan Lidral-Porter <dlp@aperiodic.org>