Intermittent unexpected CI behaviour for py27 runs
#### Code Sample, a copy-pastable example if possible

During working on PR #20437 a strange behaviour was observed concerning the CI runs:
In a first commit I've unintentionally uploaded code which did an *unconditional*
```from http.client import HTTPResponse```
in `pandas/io/excel.py`. The module `http.client` is available in py3, but not in py27, there the equivalent functionality is in module `httplib`. Nevertheless, firstly the build passed all three CI instances (CircleCI, TravisCI, AppVeyor) for py3 and for py27 without errors.

#### Problem description

Afterwards I did further commits on #20437, first on a new branch in my repo for testing and again on the PR branch. In both cases I had the same import line in `pandas/io/excel.py` with `http.client`. For these two commits, the corresponding runs failed for py27 on all CI instances as expected, on the first `import pandas`.

So far it's not clear to me how the first commit could pass on all CI instances. There seem to be some CI issue which occasionally happens and which could prevent incompatibilities from being revealed.

In order to find a module which may accidentally provide an import named `http` I wrote 
a little script (see in #20437) which collected all dependencies from the files `ci/requirements-2.7(_[A-Z]+)?\.(build|pip|run)`, but this doesn't seem to be the case.

#### Expected Output

Unconditional code which is not supported by py27 should always fail in CI for py27 runs.

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

<details>
commit: 01882ba 
python: 3.6.4.final.0 
python-bits: 64 
OS: Linux 
OS-release: 4.4.0-116-generic 
machine: x86_64 
processor: x86_64 
byteorder: little 
LC_ALL: None 
LANG: de_DE.UTF-8 
LOCALE: de_DE.UTF-8

pandas: 0.23.0.dev0+657.g01882ba
pytest: 3.4.2
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.2
IPython: 6.2.1
sphinx: 1.7.1
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
</details>
