MAINT: Remove self.assertTrue from testing
Title is self-explanatory.

Partially addresses #15990.
Appveyor failing due to network issues.  Travis and Circle seem okay.

@jreback : Could you restart the Appveyor build? yeah those are 'network' tests that don't have the decorator! (as I think there is some interaction with parametrize).  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=h1) Report
> Merging [#16158](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2d9909c62b73ea89c556aee22516cb3d0596e827?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16158/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16158      +/-   ##
==========================================
+ Coverage   90.85%   90.85%   +<.01%     
==========================================
  Files         159      159              
  Lines       50785    50785              
==========================================
+ Hits        46141    46143       +2     
+ Misses       4644     4642       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (ø)` | :arrow_up: |
| #single | `40.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.43% <0%> (+0.09%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=footer). Last update [2d9909c...ee96e02](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=h1) Report
> Merging [#16158](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2d9909c62b73ea89c556aee22516cb3d0596e827?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16158/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16158      +/-   ##
==========================================
+ Coverage   90.85%   90.85%   +<.01%     
==========================================
  Files         159      159              
  Lines       50785    50785              
==========================================
+ Hits        46141    46143       +2     
+ Misses       4644     4642       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (ø)` | :arrow_up: |
| #single | `40.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.43% <0%> (+0.09%)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=footer). Last update [2d9909c...ee96e02](https://codecov.io/gh/pandas-dev/pandas/pull/16158?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! > yeah those are 'network' tests that don't have the decorator!

Is there a reason the `tm.network` decorator is not applied? > Is there a reason the tm.network decorator is not applied?

I think I briefly tried it and couldn't get it to work when the parameterize one was as well.
give it a shot! The test is actually decorated because the function called in each test has the `tm.network` decorator.  I suspect it's because network failures are supposed to trigger test failures per the documentation of `tm.network`.