Fix DeprecationWarnings

[hugovk]

There are a couple of deprecation warnings on Python 3.7:

⌂112% [hugo:/tmp/dominate] master ± find . -name "*.pyc" -exec rm -f {} \;
⌂112% [hugo:/tmp/dominate] master ± pytest
================================================================ test session starts =================================================================
platform darwin -- Python 3.7.0, pytest-3.8.1, py-1.5.4, pluggy-0.7.1
rootdir: /private/tmp/dominate, inifile:
plugins: cov-2.5.1, flaky-3.4.0
collected 28 items

tests/test_document.py ....                                                                                                                    [ 14%]
tests/test_dom1core.py .                                                                                                                       [ 17%]
tests/test_html.py ...................                                                                                                         [ 85%]
tests/test_utils.py ....                                                                                                                       [100%]
===Flaky Test Report===


===End Flaky Test Report===

================================================================== warnings summary ==================================================================
/private/tmp/dominate/dominate/dom_tag.py:23: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import defaultdict, namedtuple, Callable

/private/tmp/dominate/dominate/util.py:87: DeprecationWarning: invalid escape sequence \d
  cc = re.compile('&(?:(?:#(\d+))|([^;]+));')

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======================================================= 28 passed, 2 warnings in 0.39 seconds ========================================================

Changes proposed in this pull request:

• Prefer collections.abc (new in Python 3.3) over collections for abstract base classes
• "In Python 3.8, the abstract base classes in collections.abc will no longer be exposed in the regular collections module. This will help create a clearer distinction between the concrete classes and the abstract base classes."
• https://docs.python.org/3.7/whatsnew/3.7.html#deprecated
• Declare the regex string as a raw string with r so the \d isn't treated as an escaped character

After

⌂92% [hugo:/tmp/dominate] fix-deprecationwarnings ± find . -name "*.pyc" -exec rm -f {} \;
⌂87% [hugo:/tmp/dominate] fix-deprecationwarnings ± pytest
================================================================ test session starts =================================================================
platform darwin -- Python 3.7.0, pytest-3.8.1, py-1.5.4, pluggy-0.7.1
rootdir: /private/tmp/dominate, inifile:
plugins: cov-2.5.1, flaky-3.4.0
collected 28 items

tests/test_document.py ....                                                                                                                    [ 14%]
tests/test_dom1core.py .                                                                                                                       [ 17%]
tests/test_html.py ...................                                                                                                         [ 85%]
tests/test_utils.py ....                                                                                                                       [100%]
===Flaky Test Report===


===End Flaky Test Report===

============================================================= 28 passed in 0.41 seconds ==============================================================

[coveralls]

Coverage decreased (-1.1%) to 95.393% when pulling cd4d482 on hugovk:fix-deprecationwarnings into a1dacee on Knio:master.