modernize syntax in Timestamp
Small cleanups to `Timestamp`+`_Timestamp`, use `@property` instead of older syntax.

The only substantive change is in `__add__`, where `result = Timestamp(normalize_date(result))` is changed to `result = result.normalize()`.  The latter has the benefits
1) is robust for tzaware timestamps,

2) does not require `normalize_date` in the namespace, so after this `Timestamp`+`_Timestamp` can be cut/paste into a new `tslibs.timestamps` and

3) is robust to subclassing of `Timestamp`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=h1) Report
> Merging [#18327](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dd38eea6116bfa21ca4db2e6621a4ce8253f927b?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18327/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18327      +/-   ##
==========================================
- Coverage   91.38%   91.36%   -0.02%     
==========================================
  Files         164      164              
  Lines       49780    49790      +10     
==========================================
+ Hits        45491    45492       +1     
- Misses       4289     4298       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.17% <ø> (ø)` | :arrow_up: |
| #single | `39.49% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.73% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95.02% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18327?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.33% <0%> (+0.05%)` | :arrow_up: |

------

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 > no, see how its used, it is for compat on creating datetime, date, Timestamp

I'll remove the comment, but I don't see why the compat usage affects the answer.  There are a handful of classmethods that in principle should use `cls` instead of a hard-coded `Timestamp`.  If `create_timestamp_from_ts` were a (cdef'd, not python-visible) classmethod, then those could be fixed/improved. > does this change any perf?

I didn't benchmark it.  The `@property` should be treated by cython identically to the older syntax.  `__repr__` might be slightly improved because the `self.freq is None` case should be more common than not, so will short-circuit that evaluation.  But that will be miniscule.

Using `result.normalize()` instead of `normalize_date(result)` is a toss-up. actually pls give this a quick benchmark, I recall this syntax as somewhat preferable, though that maybe outdated. Looks like noise:

```
asv continuous -f 1.1 -E virtualenv master HEAD -b timestamp -b timeseries
[...]
       before           after         ratio
     [6405919e]       [fcaf3c33]
+     5.75±0.01μs      7.41±0.01μs     1.29  timestamp.TimestampProperties.time_days_in_month
+     5.89±0.02μs      7.30±0.01μs     1.24  timestamp.TimestampProperties.time_is_month_start
+         443±2ms         544±30ms     1.23  timeseries.ToDatetime.time_format_no_exact
+     18.5±0.09μs       21.3±0.1μs     1.15  timeseries.Offsets.time_custom_bday_incr
+      17.5±0.2μs         19.7±1μs     1.12  timeseries.Offsets.time_timeseries_day_apply
+      392±0.06ms       437±0.03ms     1.12  timeseries.DatetimeIndex.time_add_offset_slow
-         116±3ns        105±0.2ns     0.90  timestamp.TimestampProperties.time_microsecond
-         447±2ms        401±0.8ms     0.90  timeseries.SemiMonthOffset.time_end_incr_rng
-        22.2±1μs       19.7±0.1μs     0.89  timeseries.Offsets.time_timeseries_day_incr
-        14.4±1μs      12.7±0.03μs     0.88  timeseries.SemiMonthOffset.time_begin_apply
-      6.39±0.3μs      5.62±0.01μs     0.88  timestamp.TimestampProperties.time_quarter
-      24.9±0.6μs       21.5±0.1μs     0.86  timeseries.SemiMonthOffset.time_begin_decr_n
-        116±20ms         33.9±3ms     0.29  timeseries.DatetimeIndex.time_dti_factorize
-        444±10ms         102±20ms     0.23  binary_ops.TimeseriesTZ.time_timestamp_ops_diff2
-        129±10ms         29.0±1ms     0.23  timeseries.DatetimeIndex.time_dti_tz_factorize
-           2.00s          406±4ms     0.20  timeseries.AsOfDataFrame.time_asof
-       929±100ms         169±30ms     0.18  binary_ops.Timeseries.time_timestamp_ops_diff2
-         388±7ms         69.1±2ms     0.18  binary_ops.TimeseriesTZ.time_timestamp_ops_diff1
```

and with affinity

```
taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timestamp -b timeseries
[...]
       before           after         ratio
     [6405919e]       [fcaf3c33]
+     5.69±0.02μs      6.68±0.01μs     1.17  timestamp.TimestampProperties.time_is_quarter_end
+     17.5±0.08μs       20.1±0.2μs     1.15  timeseries.Offsets.time_timeseries_day_apply
+     29.8±0.09μs       34.1±0.2μs     1.15  timeseries.Offsets.time_custom_bday_decr
+     5.89±0.02μs      6.50±0.02μs     1.10  timestamp.TimestampProperties.time_is_year_start
-     20.7±0.07μs      18.8±0.05μs     0.91  timeseries.SemiMonthOffset.time_begin_decr
-      27.9±0.1μs       24.3±0.1μs     0.87  timeseries.Offsets.time_custom_bday_cal_incr
-       53.4±10ms         26.6±2ms     0.50  timeseries.DatetimeIndex.time_dti_factorize
``` your asv runs are still very wild. thanks