added getconnectionstring()

[hsum]

added function to generate a connection string when provided same arguments as getpass.

def getconnectionstring(
    host=DEFAULT_HOST,
    port=DEFAULT_PORT,
    dbname=DEFAULT_DBNAME,
    user=DEFAULT_USER,
):

If getpass() returns '', getconnectionstring returns ''

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9b55ed5). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #6   +/-   ##
=========================================
  Coverage          ?   98.78%           
=========================================
  Files             ?        1           
  Lines             ?       82           
  Branches          ?       13           
=========================================
  Hits              ?       81           
  Misses            ?        0           
  Partials          ?        1

Impacted Files	Coverage Δ
pgpasslib.py	98.78% <100%> (ø)

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