f-strings vs @ syntax in `.query` call? A suggestion for a deprecation
This is not a problem but an observation and possibly something for a deprecation (as discussed with @datapythonista at PyDataLondon this weekend).

Pandas supports a `.query` syntax including the `.query("my_col < @some_var")` syntax to bring in a local `some_var`. This predates Python's f-string syntax which was introduced in Python 3.6. 

I wonder if Python's f-string syntax is a more standard way to tackle variable substitution going forwards? The equivalent would be `.query(f"my_col < {some_var}")`.

The benefit would be to reduce the amount of code that's custom to Pandas and to stick to a more-standard (IMHO) approach to variable substitution.

#### Code Sample, a copy-pastable example if possible

```python
In [4]: df = pd.DataFrame({'col': [1, 2, 3, 4]})
In [5]: value = 3

In [6]: df.query("col < @value")
Out[6]: 
   col
0    1
1    2

In [7]: df.query(f"col < {value}")
Out[7]: 
   col
0    1
1    2
```

#### Output of ``pd.show_versions()``

<details>

In [10]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Linux
OS-release: 5.0.0-19-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.4
...
</details>
