input: Write a function to calculate the correlation coefficient between two variables from a given CSV file.
  assert np.isclose(calculate_correlation('./data.csv', 'variable1', 'variable2'), 0.85, atol=0.01)
  assert np.isclose(calculate_correlation('./data.csv', 'variable3', 'variable4'), -0.75, atol=0.01)
  assert np.isclose(calculate_correlation('./data.csv', 'variable5', 'variable6'), 0.0, atol=0.01)
output: import pandas as pd
  import numpy as np
  
  def calculate_correlation(file_path, var1, var2):
      data = pd.read_csv(file_path)
      correlation_matrix = data[[var1, var2]].corr()
      correlation = correlation_matrix.loc[var1, var2]
      return correlation