requests package requirement not listed

[wholmgren]

Describe the bug
import pvlib fails due to unspecified requests dependency:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/holmgren/miniconda3/envs/pvlibtest/lib/python3.7/site-packages/pvlib/__init__.py", line 9, in <module>
    from pvlib import iotools
  File "/Users/holmgren/miniconda3/envs/pvlibtest/lib/python3.7/site-packages/pvlib/iotools/__init__.py", line 13, in <module>
    from pvlib.iotools.psm3 import get_psm3  # noqa: F401
  File "/Users/holmgren/miniconda3/envs/pvlibtest/lib/python3.7/site-packages/pvlib/iotools/psm3.py", line 8, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'

To Reproduce
Steps to reproduce the behavior:

1. conda create -n pvlibtest python=3.7
2. pip install pvlib
3. python -c import pvlib

Expected behavior
pip install pvlib works from a clean environment.

Versions:

• pvlib.__version__: 0.6.2
• pandas.__version__: 0.24.2
• python: 3.7

Additional context
#694 added a dependency on requests. The CI environments pull in requests through some of their dependencies, so it wasn't caught there. Should make a new issue for adding CI tests for pip installs into a clean environment.

0.6.3 coming up...

[mikofski]

oops! thanks for catching and quick fix!