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>

Lib/pdb.py

@@ -160,16 +160,14 @@ class Pdb(bdb.Bdb, cmd.Cmd):
         self.allow_kbdint = False
         self.nosigint = nosigint
 
-        # Read $HOME/.pdbrc and ./.pdbrc
+        # Read ~/.pdbrc and ./.pdbrc
         self.rcLines = []
         if readrc:
-            if 'HOME' in os.environ:
-                envHome = os.environ['HOME']
-                try:
-                    with open(os.path.join(envHome, ".pdbrc")) as rcFile:
-                        self.rcLines.extend(rcFile)
-                except OSError:
-                    pass
+            try:
+                with open(os.path.expanduser('~/.pdbrc')) as rcFile:
+                    self.rcLines.extend(rcFile)
+            except OSError:
+                pass
             try:
                 with open(".pdbrc") as rcFile:
                     self.rcLines.extend(rcFile)