math/py-pandas-datareader: Update to 0.7.4

• Pet portlint by making the USES block separate while I'm here.

Changelog since 0.7.0:

• Restore Python 2.7 compatibility since the 0.7.x branch
• Fixed import of pandas.compat dependencies

https://github.com/pydata/pandas-datareader/compare/v0.7.0...v0.7.4

PR: 239591
Submitted by: Shane <FreeBSD@ShaneWare.Biz> (maintainer)
MFH: 2019Q3