Require arrow>=0.15.6 (#380)

Since its last release[0], the heavily used arrow library supports ISO
week dates[1]. The CLI tests assumed that arrow does not support this.
However, the new version nullified this assumption.

As a provisional measure, the version of arrow was limited upwards in
PR #372[2]. An obsolete version was thereby requested.

A current version, or at least support for one, is important for
third-party package managers. Especially GNU/Linux distributions prefer
to use their own package manager to install software over pip.

Thus, this commit removes both the restriction to an outdated arrow
version in the requirements.txt and validates previously invalid marked
week dates.

[0]:https://github.com/crsmithdev/arrow/releases/tag/0.15.6
[1]:https://en.wikipedia.org/wiki/ISO_week_date
[2]:#372

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Require latest Arrow version 0.15.6 to support ISO week dates (#380)
+
 ## [1.9.0] - 2020-05-27
 
 ### Added

requirements.txt

@@ -1,5 +1,5 @@
-arrow!=0.11,!=0.12.0,<0.15.6
+arrow>=0.15.6
 click>=7.0
 click-didyoumean
 colorama; sys_platform == "win32"
-requests
+requests

tests/test_cli.py

@@ -44,6 +44,10 @@
         .replace(hour=14, minute=5, second=0)
         .format('YYYY-MM-DD HH:mm:ss')
     ),
+    ('2018-W08', '2018-02-19 00:00:00'),  # week dates
+    ('2018W08', '2018-02-19 00:00:00'),
+    ('2018-W08-2', '2018-02-20 00:00:00'),
+    ('2018W082', '2018-02-20 00:00:00'),
 ]
 
 INVALID_DATES_DATA = [
@@ -52,10 +56,6 @@
     ('201804'),
     ('18-04-10'),
     ('180410'),  # truncated representation not allowed
-    ('2018-W08'),  # despite week dates being part of ISO-8601
-    ('2018W08'),
-    ('2018-W08-2'),
-    ('2018W082'),
     ('hello 2018'),
     ('yesterday'),
     ('tomorrow'),