tslib flake8 cleanup, missing declarations
State: closed

replace `isinstance` checks with the faster C versions
change cpdef-->cdef for internal field-getter methods
remove get_named_field in favor of faster dict lookup (yah yah I'll show asv results in a bit)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Labels:
name: Datetime
description: Datetime data dtype
name: Timedelta
description: Timedelta data type
name: Code Style
description: Code style, linting, code_checks

Comments:
> can you show updated benchmarks (for things that changed)

Should just be `weekday_name` property.

Old:
```
In [2]: ts = pd.Timestamp.now()
In [3]: ts2 = ts.tz_localize('Europe/Amsterdam')
In [4]: %timeit ts.weekday_name
10000 loops, best of 3: 79.5 µs per loop
In [5]: %timeit ts2.weekday_name
10000 loops, best of 3: 150 µs per loop
```

New:
```
In [2]: ts = pd.Timestamp.now()
In [3]: ts2 = ts.tz_localize('Europe/Amsterdam')
In [4]: %timeit ts.weekday_name
The slowest run took 11.97 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 4.92 µs per loop
In [5]: %timeit ts2.weekday_name
100000 loops, best of 3: 5.26 µs per loop
```

since it looks like you are doing flake8 fixups, can you edit the lint.sh script to pick these things up?
if some things are currently failing, just separate those out into a separate section (in the same scripts) with a comment and we can uncomment later.
> can you edit the lint.sh script to pick these things up?

Just added this to the TODO list. 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=h1) Report
> Merging [#17907](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5bf7f9a4ff6968fd444fa7098f8dc95586591994?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/17907/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17907      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50105    50112       +7     
==========================================
- Hits        45715    45713       -2     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (ø)` | :arrow_up: |
| #single | `40.31% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.1% <0%> (-2.95%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.18% <0%> (-1.63%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.35% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.14% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.58% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.03% <0%> (+0.05%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/17907?src=pr&el=tree-more) | |

------

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thanks @jbrockmendel