A list comprehension could be helpful here. You can try something like:

hex_values = ['#b4a795', '#e7e0d6', '#d7cfc8']

hsv_values = [rgb2hsv(*hex2rgb(hex_val)) for hex_val in hex_values]

The problem is in your dec2hex code:

def dec2hex(d):
    """return a two character hexadecimal string representation of integer d"""
    r = "%X" % d
    return r if len(r) > 1 else r+r

When your value is less than 16, you're duplicating it to get the value, in other words, multiplying it by 17. You want this:

def dec2hex(d):
    """return a two character hexadecimal string representation of integer d"""
    return "%02X" % d

I would use regular expressions:

regex = r'hsl\(\s*(\d+),\s*(\d+)%,\s*(\d+)%\s*\);'
lines = [re.findall(regex,line) for line in one_dark_syn.split('\n')]
rgbs = [colorsys.hsv_to_rgb(int(line[0][0])/360,
                            int(line[0][1])/100,
                            int(line[0][2])/100) for line in lines if line]
# [(0.2915, 0.519842, 0.55), (0.1188, 0.41646, 0.66), (0.5762, 0.268, 0.67), 
#  (0.48257, 0.62, 0.3844), (0.65, 0.2275, 0.2627), (0.51, 0.2856, 0.2652), 
#  (0.61, 0.43981, 0.2806), (0.69, 0.528195, 0.2277)]

Finally, convert RGB triplets to a HEX string:

rgbhex = ["".join("%02X" % round(i*255) for i in rgb) for rgb in rgbs]
# ['1D3437', '0C2A42', '3A1B43', '303E26', '41171A', '331D1B', '3D2C1C', '453517']