py27 loss of precision

[crusaderky]

There are precision issues in python 2.7:

>>> ureg.define("mylen = 1.000000000001 m")
>>> ureg.Quantity(1, "mylen").to("m")
<Quantity(1.0, 'meter')>

In Python 3:

1.000000000001 <Unit('meter')>

This is caused by incorrectly using str(float) instead of repr(float) across the codebase. In Python 2.7, the two are not the same:

>>> str(1.000000000001)
'1.0'
>>> repr(1.000000000001)
'1.000000000001'

I noticed this issue after my eye fell on one of these cases:

pint/pint/registry.py

Lines 751 to 754 in c777e3c

	if isinstance(value, Decimal):
	factor = Decimal(str(factor))
	elif isinstance(value, Fraction):
	factor = Fraction(str(factor))

Personal note: I hold no interest in Python 2.7 so I'm afraid I'm not volunteering to fix this. I strongly suggest canning support for Python < 3.5 ASAP.