Use raw string when invalid escape sequence present.

Those emit deprecation warnings at import/compile time so unfortunately
we can't trap that with iptest as modules are already imported.

The way to test that is to start IPython via `python -We -m IPython`,
and we should try to likely do that in travis.

But first fix all places that raise an error

IPython/core/debugger.py

@@ -176,7 +176,7 b' class Tracer(object):'
         self.debugger.set_trace(sys._getframe().f_back)
 
 
-RGX_EXTRA_INDENT = re.compile('(?<=\n)\s+')
+RGX_EXTRA_INDENT = re.compile(r'(?<=\n)\s+')
 
 
 def strip_indentation(multiline_string):

IPython/utils/path.py

@@ -202,7 +202,7 b' def get_home_dir(require_writable=False):'
                 import _winreg as wreg  # Py 2
             key = wreg.OpenKey(
                 wreg.HKEY_CURRENT_USER,
-                "Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders"
+                r"Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders"
             )
             homedir = wreg.QueryValueEx(key,'Personal')[0]
             key.Close()

IPython/utils/text.py

@@ -588,7 +588,7 b' class DollarFormatter(FullEvalFormatter):'
         In [4]: f.format('$a or {b}', a=1, b=2)
         Out[4]: '1 or 2'
     """
-    _dollar_pattern_ignore_single_quote = re.compile("(.*?)\$(\$?[\w\.]+)(?=([^']*'[^']*')*[^']*$)")
+    _dollar_pattern_ignore_single_quote = re.compile(r"(.*?)\$(\$?[\w\.]+)(?=([^']*'[^']*')*[^']*$)")
     def parse(self, fmt_string):
         for literal_txt, field_name, format_spec, conversion \
                     in Formatter.parse(self, fmt_string):