Address four deprecation warnings (#375)

tests/formatting/test_formatter.py:241
  /home/altendky/pendulum/tests/formatting/test_formatter.py:241:
DeprecationWarning: invalid escape sequence \[
    assert f.format(d, "[YYYY] YYYY \[YYYY\]") == "YYYY 2016 [2016]"

tests/formatting/test_formatter.py:242
  /home/altendky/pendulum/tests/formatting/test_formatter.py:242:
DeprecationWarning: invalid escape sequence \D
    assert f.format(d, "\D D \\\D") == "D 28 \\28"

tests/datetime/test_from_format.py::test_from_format_with_escaped_elements
  /home/altendky/pendulum/pendulum/formatting/formatter.py:406:
DeprecationWarning: Flags not at the start of the expression
'(?P<YYYY>\\d{1,4}|\\d{' (truncated)
    if not re.match(pattern, time):

tests/datetime/test_from_format.py::test_from_format[Thursday 25th
December 1975 02:15:16 PM -05:00-dddd Do MMMM YYYY hh:mm:ss A
Z-1975-12-25T14:15:16-05:00-None]
  /home/altendky/pendulum/pendulum/formatting/formatter.py:406:
DeprecationWarning: Flags not at the start of the expression
'(?P<dddd>Sunday|Mond' (truncated)
    if not re.match(pattern, time):

Author: altendky

pendulum/formatting/formatter.py

@@ -25,8 +25,8 @@
 _MATCH_5_TO_6 = "\d{5}\d?"
 _MATCH_UNSIGNED = "\d+"
 _MATCH_SIGNED = "[+-]?\d+"
-_MATCH_OFFSET = "(?i)Z|[+-]\d\d:?\d\d"
-_MATCH_SHORT_OFFSET = "(?i)Z|[+-]\d\d(?::?\d\d)?"
+_MATCH_OFFSET = "[Zz]|[+-]\d\d:?\d\d"
+_MATCH_SHORT_OFFSET = "[Zz]|[+-]\d\d(?::?\d\d)?"
 _MATCH_TIMESTAMP = "[+-]?\d+(\.\d{1,6})?"
 _MATCH_WORD = (
     "(?i)[0-9]*"

tests/formatting/test_formatter.py

@@ -238,8 +238,8 @@ def test_date_formats():
 def test_escape():
     f = Formatter()
     d = pendulum.datetime(2016, 8, 28)
-    assert f.format(d, "[YYYY] YYYY \[YYYY\]") == "YYYY 2016 [2016]"
-    assert f.format(d, "\D D \\\D") == "D 28 \\28"
+    assert f.format(d, r"[YYYY] YYYY \[YYYY\]") == "YYYY 2016 [2016]"
+    assert f.format(d, r"\D D \\D") == "D 28 \\28"
 
 
 def test_date_formats_missing():