from urllib.request import urlopen

# Get data as string
x = urlopen('https://www.nsstc.uah.edu/data/msu/v6.0/tlt/uahncdc_lt_6.0.txt').read().decode('utf-8')

# Split data by line
x = x.split('\n')

# Remove non-data lines
x = x[1:539]

# Split data by whitespace
x = [y.split() for y in x]

# Enumerate over Jan-Sep in years >= 2015
x = tuple((i - 433, float(y[2])) for i, y in enumerate(x) if int(y[0]) >= 2015 and int(y[1]) < 10)

# Now plot this data with your tool of choice
print(x)