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']

You need to make sure to normalize the values according to the matching ranges. According to your values (and the range hints in this website), it seems you're using values of H in the range [0, 360] ; S and V in the range [0, 100]. On the other hand, the built-in library colorsys uses all [0, 1] ranges. So a bit of normalization will give you the required results:

import colorsys

def hsv_to_hsl(hsv):
    h, s, v = hsv
    rgb = colorsys.hsv_to_rgb(h/360, s/100, v/100)
    r, g, b = rgb
    h, l, s = colorsys.rgb_to_hls(r, g, b)
    return h*360, s*100, l*100

hsv = (300, 65, 40)

print(hsv)
print(hsv_to_hsl(hsv))

Now gives the expected:

(300, 65, 40)
(300.0, 48.148148148148145, 27.0)

If you are looking for a manual implementation, you can use the algorithm described here (now the values are: H in range [0, 360] and S,V and L are in [0, 1]):

def hsv_to_hsl(hsv):
    h, s, v = hsv
    l = v * (1 - s/2)
    s = 0 if l in (0, 1) else (v - l)/min(l, 1-l)
    return h, s, l

def hsl_to_hsv(hsl):
    h, s, l = hsl
    v = l + s * min(l, 1-l)
    s = 0 if v == 0 else 2*(1 - l/v)
    return h, s, v

To compare the result with colorsys:

h, s, l = (300, 48.148, 27)

custom = hsl_to_hsv((h, s/100, l/100))
print("Custom function: ", custom[0], custom[1]*100, custom[2]*100)
expected = colorsys.rgb_to_hsv(*colorsys.hls_to_rgb(h/360, l/100, s/100))
print("Expected result: ", expected[0]*360, expected[1]*100, expected[2]*100)

Indeed gives:

Custom function:  300 64.99986499986498 39.99996
Expected result:  300.0 64.999864999865 39.99996

Use python std colorsys as HSL -> RGB -> HSV:

import colorsys
c = {'h': 300, 's': 67.0, 'l': 80.0}
r, g, b = colorsys.hls_to_rgb(c["h"], c["l"], c["s"])
h, s, v = colorsys.rgb_to_hsv(r, g, b)

Resu

Converting HSV to RGB requires a formula - Python, actually has had it in its standard library since the dawn of time - there is the colorsys module in the stdlib.

However, it works with floating point values in the 0.0-1.0 range - so, some code is needed if you want to convert your component values as integers.

Of course, just import colorsys does nothing - although possible, almost no package will have a global side-effect just by being imported (except from __future__ import .... statements which affect the way the Python code itself is compiled). You have to call the functions inside it.

That said, I think this could be your function (give or take "1" at the extreme boundaries:

import colorsys

def hsv_to_hex(h,s,v):
    r, g, b = (int(255 * component) for component in colorsys.hsv_to_rgb(h / 255, s / 255, v / 255))
    return f"#{r:02x}{g:02x}{b:02x}"

I am assuming the values you have for the HSV components are normalized in the 0-255 range. There is another, more usual convention, where they are 0-360 for H and 0-100 for S and V - change the divisors accordingly in the function above. (You have 172 for S, so it can´t be in the 0-100 range)