Second attempt at fixing timezone test issues

This is a followup to #10. It turns out that `freezegun` does not yet
support `dateutil`'s `tzlocal`, which is used by `timew-report`
[here]. [This comment] on the `freezegun` repository provides a context
manager solution that can be used for now. This change properly fixes #6.

[here]: https://github.com/lauft/timew-report/blob/c972357a44640c7a5f803d79fc77ca597e1b22f0/timewreport/interval.py#L49
[This comment]: https://github.com/lauft/timew-report/blob/c972357a44640c7a5f803d79fc77ca597e1b22f0/timewreport/interval.py#L49

Pipfile

@@ -8,7 +8,6 @@ timew-report = "<1.3"
 
 [dev-packages]
 pytest = "*"
-pytest-freezegun = "*"
 
 [requires]
 python_version = "3.6"

tests/test_invoice.py

@@ -1,3 +1,6 @@
+import contextlib
+import os
+import time
 import unittest
 from datetime import datetime, timedelta, timezone
 from random import randint
@@ -11,6 +14,21 @@
 from billwarrior.invoice import Invoice, ItemCategory, LineItem
 
 
+@contextlib.contextmanager
+def mock_tz(new_tz):
+    old_tz = os.environ.get('TZ')
+    os.environ['TZ'] = new_tz
+    time.tzset()
+    try:
+        yield
+    finally:
+        if old_tz is not None:
+            os.environ['TZ'] = old_tz
+        else:
+            del os.environ['TZ']
+        time.tzset()
+
+
 class BillWarriorConfigFake(BillWarriorConfig):
     def __init__(self):
         pass
@@ -57,28 +75,28 @@ def test_creates_different_categories_from_interval_tags_and_mapping(self):
         self.assertEqual(str(expected_a), str(items[0]))
         self.assertEqual(str(expected_b), str(items[1]))
 
-    @pytest.mark.freeze_time(tz_offset=2)
     def test_groups_intervals_of_same_category(self):
-        same_day = datetime.today()
-        a, b, c = (
-            tests.give_interval(same_day, tags=["on-site", "coding"]),
-            tests.give_interval(same_day, tags=["coding", "off-site"]),
-            tests.give_interval(tags=["coding", "cafe"]),
-        )
+        with mock_tz('UTC'):
+            same_day = datetime.today()
+            a, b, c = (
+                tests.give_interval(same_day, tags=["on-site", "coding"]),
+                tests.give_interval(same_day, tags=["coding", "off-site"]),
+                tests.give_interval(tags=["coding", "cafe"]),
+            )
 
-        billw_config = BillWarriorConfigFake.build({"coding": "Consulting & Research"})
+            billw_config = BillWarriorConfigFake.build({"coding": "Consulting & Research"})
 
-        invoice = Invoice([a, b, c], billw_config)
-        items = invoice.items()
+            invoice = Invoice([a, b, c], billw_config)
+            items = invoice.items()
 
-        expected = ItemCategory(
-            "Consulting & Research",
-            {same_day.date(): [a, b], c.get_date().date(): [c]},
-            0.0,
-        )
+            expected = ItemCategory(
+                "Consulting & Research",
+                {same_day.date(): [a, b], c.get_date().date(): [c]},
+                0.0,
+            )
 
-        self.assertEqual(len(items), 1)  # one category
-        self.assertEqual(str(items[0]), str(expected))
+            self.assertEqual(len(items), 1)  # one category
+            self.assertEqual(str(items[0]), str(expected))
 
     def test_raises_exception_when_interval_sorts_into_more_than_one_category(self):
         a, b = (
@@ -228,17 +246,18 @@ def test_items_omitted_when_unit_price_is_negative(self):
         self.assertEqual(str(category), "")
 
     def test_line_items_should_be_populated_with_entries_per_day(self):
-        a_day = datetime.today()
-        entries = [
-            [tests.give_interval(a_day + timedelta(days=1))],
-            [tests.give_interval(a_day + timedelta(days=2))],
-            [tests.give_interval(a_day + timedelta(days=3))],
-        ]
-        intervals_by_day = {entry[0].get_date().date(): entry for entry in entries}
+        with mock_tz('UTC'):
+            a_day = datetime.today()
+            entries = [
+                [tests.give_interval(a_day + timedelta(days=1))],
+                [tests.give_interval(a_day + timedelta(days=2))],
+                [tests.give_interval(a_day + timedelta(days=3))],
+            ]
+            intervals_by_day = {entry[0].get_date().date(): entry for entry in entries}
 
-        category = ItemCategory("arbitray category", intervals_by_day, 0.0)
+            category = ItemCategory("arbitray category", intervals_by_day, 0.0)
 
-        self.assertEqual(len(category.line_items), len(entries))
+            self.assertEqual(len(category.line_items), len(entries))
 
     def test_line_items_should_be_sorted_by_date(self):
         entries = [

tests/testsupport.py

@@ -15,4 +15,3 @@ def give_interval(day=None, tags=[]):
     end = start + timedelta(0, randint(60 * 5, 60 * 60 * 2))  # up to 2h
 
     return TimeWarriorInterval(start.strftime(DT_FORMAT), end.strftime(DT_FORMAT), tags)
-