You have to clone the tag ("version") that you are looking for, and only that one defining the branch and avoiding any further depth (is this is your case).
For example, to clone version 0.6.1 to your local folder "pvlib_0.6.1" to distinguish between versions:
$ git clone --branch v0.6.1 --depth 1 https://github.com/pvlib/pvlib-python.git ./pvlib_0.6.1
Now, you can import that version including in sys path on the first position by:
$ python
>>> import sys
>>> sys.path.insert(0, './pvlib_0.6.1') # Or full path if you store it in another place
>>> import pvlib
>>> pvlib.__version__
u'0.6.1'
You cannot call multiple versions in the same script as your first proposal, but you can do something like:
$ python
>>> DISTNAME = '0.6.1'
>>> import sys
>>> sys.path.insert(0, './pvlib_%s' % DISTNAME)
>>> import pvlib
>>> pvlib.__version__
u'0.6.1'
Remember the position of the inserted path must be in position 0, if not and you already installed a pvlib version with pip in your system, when you import the library, the system could be loaded with priority respect to other version that you might want to use.