PERF: ints_to_pydatetime
This refactors ints_to_pydatetime to use a much more concise pattern (similar to #35077 but without the helper function refactored out) that I intend to move all of the ~7 functions using get_dst_info to use.  In the process, we are slower on very small arrays and faster on bigger arrays, which I think is a good trade:

```
% asv continuous -E virtualenv -f 1.01 master HEAD -b time_ints_to_pydatetime
[...]
       before           after         ratio
     [a4e19fa5]       [2492aeb4]
     <master>         <ref-ints_to_pydatetime-3>
+     3.95±0.09μs       8.22±0.5μs     2.08  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 0, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
+      4.80±0.2μs       9.53±0.5μs     1.99  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 0, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
+      5.45±0.3μs       9.01±0.2μs     1.65  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 0, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
+      5.31±0.6μs       8.31±0.3μs     1.57  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 0, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
+        6.08±1μs       8.84±0.2μs     1.45  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 0, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
+      7.00±0.3μs       10.0±0.3μs     1.44  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 1, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
+      7.17±0.1μs       10.3±0.3μs     1.43  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 1, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
+      7.47±0.1μs       9.77±0.2μs     1.31  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 1, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
+      7.08±0.2μs       8.46±0.5μs     1.20  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 1, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      10.7±0.6μs       9.48±0.3μs     0.88  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 0, datetime.timezone(datetime.timedelta(seconds=3600)))
-     3.38±0.02μs      2.89±0.06μs     0.85  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('date', 0, None)
-        74.3±1μs         56.3±1μs     0.76  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 100, datetime.timezone.utc)
-        73.4±4μs         54.7±2μs     0.74  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 100, None)
-        20.7±3ms         9.31±2ms     0.45  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 10000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      15.4±0.2ms         6.23±1ms     0.40  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 10000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      1.75±0.03s          699±5ms     0.40  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 1000000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      1.56±0.05s         614±20ms     0.39  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 1000000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      1.65±0.01s         650±10ms     0.39  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 1000000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-        156±10μs         57.3±9μs     0.37  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 100, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-        16.8±1ms       5.95±0.2ms     0.36  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 10000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-         164±7μs       58.3±0.8μs     0.36  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 100, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      14.8±0.1ms       5.14±0.5ms     0.35  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 10000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-      1.42±0.02s         491±20ms     0.34  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('datetime', 1000000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-         169±6μs         56.0±3μs     0.33  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 100, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-        193±30μs       63.1±0.9μs     0.33  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 100, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-        229±30μs         73.7±3μs     0.32  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('timestamp', 100, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      1.65±0.06s         505±20ms     0.31  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 1000000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-         159±3μs         46.1±2μs     0.29  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 100, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-        16.0±1ms       4.64±0.1ms     0.29  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 10000, <DstTzInfo 'US/Pacific' LMT-1 day, 16:07:00 STD>)
-      15.2±0.2ms       3.78±0.2ms     0.25  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 10000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
-       1.65±0.1s         408±20ms     0.25  tslibs.tslib.TimeIntsToPydatetime.time_ints_to_pydatetime('time', 1000000, tzfile('/usr/share/zoneinfo/Asia/Tokyo'))
```
very nice, cleaning up some nasty code + perf!
 > cleaning up some nasty code 

Yep.  #35108, #35106, #35104, and #35102 together fix our basically-incorrect tz_convert_single usage, at which point we can clean up a bunch more. i merged everything above (i think)
pls give a check 