BigQuery tests should avoid `pandas` in Python 3.4

[dhermes]

At the end of the 0.20.x series 3.4 was supported:

but at the beginning of the 0.21.x series it is not:

There didn't seem to be an issue with 0.21.0 but 0.21.1 just came out and broke the first build it ran in.

[theacodes]

I guess this begs the question of if we should drop 3.4 support.

[dhermes]

Heck yeah we should. Hasn't it already been EOL-ed?

[chemelnucfin]

https://devguide.python.org/#status-of-python-branches

Apparently not until 2019-03-16.

[tseaver]

3.3 was EOLed in September. FWIW, 3.4 is still the default Python 3 on LTS Linux versions.

[tswast]

Sounds like they dropped it because Conda doesn't support it. pandas-dev/pandas#15251 That doesn't really apply to us here.