Bug fix/560: YY maps 2018 to 20

[dyerd]

Potential bug fix for issues mentioned in #560. Enforces that year given for 'DD MMMM YY' is only 2 digits, and that year given for 'DD MMMM YYYY' is only 4 digits.

Per the comments on the issue report:

arrow.get('12 October 2018', 'DD MMMM YY') now raises an exception

and

arrow.get('01 January 123456789101112', 'DD MMMM YYYY') now raises an exception

[codecov-io]

Codecov Report

Merging #580 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #580   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines        3399   3407    +8     
  Branches      238    239    +1     
=====================================
+ Hits         3399   3407    +8

Impacted Files	Coverage Δ
arrow/arrow.py	100% <ø> (ø)	⬆️
tests/parser_tests.py	100% <100%> (ø)	⬆️
arrow/parser.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 43ffd2a...e58f0dc. Read the comment docs.

[systemcatch]

Hey @dyerd thanks for the contribution. I'm just testing this out and making sure there aren't snags in implementing this.

[systemcatch]

arrow.get('12 October 2018', 'DD MMMM YY') => ParserError ✔️

arrow.get('01 January 123456789101112', 'DD MMMM YYYY') => ParserError ✔️

arrow.get("15/01/2019", ["D/M/YY","D/M/YYYY"]) => <Arrow [2019-01-15T00:00:00+00:00]> ✔️

arrow.get("15/01/19", "D/M/YY") => <Arrow [2019-01-15T00:00:00+00:00]> ✔️

arrow.get('12 October 2018 -0700', 'DD MMMM YYYY Z') => <Arrow [2018-10-12T00:00:00-07:00]> ✔️

arrow.get("68096653015/01/19", "YY/M/DD") => <Arrow [2015-01-19T00:00:00+00:00]> ❌

Seems likes it will still wrongly match if YY is at the start of the string.

[jadchaar]

Closing this PR as this bug has been fully resolved in the Version-0.15.0 branch.