TestSQLApi/TestSQLApiConn::test_database_uri_string errors if pg8000 is installed

[neirbowj]

Code Sample, a copy-pastable example if possible

db_uri = "postgresql+pg8000://user:pass@host/dbname"
with tm.assert_raises_regex(ImportError, "pg8000"):
    sql.read_sql("select * from table", db_uri)

Problem description

The test code comments specify that pg8000 is a "driver that will not be installed on Travis". However, when the test suite is run elsewhere, that driver may be installed.

Expected Output

The test should pass or skip in any environment where pandas is expected to run successfully.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 2.7.13.final.0 python-bits: 64 OS: FreeBSD OS-release: 10.3-STABLE machine: amd64 processor: amd64 byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.20.1
pytest: 3.1.0
pip: None
setuptools: 32.1.0
Cython: 0.25.2
numpy: 1.11.2
scipy: 0.19.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.0.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 1.5.3
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: 3.6.0
bs4: 4.5.1
html5lib: 0.9999999
sqlalchemy: 1.1.10
pymysql: 0.7.11.None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None

[neirbowj]

I looked for a way to get SQLAlchemy to enumerate the currently supported drivers and came up dry. In order to skip this test less often, I think it would be necessary to hard code a list of known drivers, which would need to be maintained, try importing them in turn to find one that isn't installed, and skip only in the unlikely event that all drivers are available. That seemed out of proportion, so I elected to just skip if pg8000 is present.