BUG: fix to_datetime failing to raise on mixed tznaive/tzaware datetimes
- [x] closes #24569
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=h1) Report
> Merging [#24663](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4bf3a0e11d2e98afc9ee76dbe54a667410d0220e?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/24663/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24663      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52382    52382              
==========================================
+ Hits        48388    48389       +1     
+ Misses       3994     3993       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24663/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24663?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/24663?src=pr&el=footer). Last update [4bf3a0e...a910a33](https://codecov.io/gh/pandas-dev/pandas/pull/24663?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/24663?src=pr&el=h1) Report
> Merging [#24663](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4bf3a0e11d2e98afc9ee76dbe54a667410d0220e?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/24663/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24663      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52382    52382              
==========================================
+ Hits        48388    48389       +1     
+ Misses       3994     3993       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24663/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24663?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/24663?src=pr&el=footer). Last update [4bf3a0e...72e46b2](https://codecov.io/gh/pandas-dev/pandas/pull/24663?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This is good to go? Do the updates to the tests satisfy 
https://github.com/pandas-dev/pandas/pull/24663/files#r245769417 @jschendel? thanks @jbrockmendel  @jschendel I think your concerns are satisfied but if not pls raise an issue / PR.