Remove invalid escape sequences to get rid of Py3 DeprecationWarning

[HarrisonGregg]

The backslashes seem unnecessary and they're causing deprecation warnings that look like:
pyasn1/type/constraint.py:130: DeprecationWarning: invalid escape sequence *

[codecov-io]

Codecov Report

Merging #161 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #161   +/-   ##
=======================================
  Coverage   85.64%   85.64%           
=======================================
  Files          29       29           
  Lines        4076     4076           
=======================================
  Hits         3491     3491           
  Misses        585      585

Impacted Files	Coverage Δ
pyasn1/type/namedval.py	75.28% <ø> (ø)	⬆️
pyasn1/type/constraint.py	89.72% <ø> (ø)	⬆️

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[etingof]

Thank you!