correctly proxy PreparedStatement non-async methods

[shamrin]

This PR lets you use prepared statement methods like .get_statusmsg() and .get_query():

stmt = await conn("select version()")
await stmt.fetch()
stmt.get_statusmsg() # => 'SELECT 1'
stmt.get_query() # => 'select version()'

[codecov[bot]]

Codecov Report

Merging #14 (d2841b0) into master (8fe8417) will increase coverage by 0.57%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   98.60%   99.17%   +0.57%     
==========================================
  Files           6        6              
  Lines         358      363       +5     
  Branches       14       14              
==========================================
+ Hits          353      360       +7     
+ Misses          2        1       -1     
+ Partials        3        2       -1     

Impacted Files	Coverage Δ
triopg/_tests/test_triopg.py	100.00% <100.00%> (ø)
triopg/_triopg.py	98.03% <100.00%> (+1.30%)	:arrow_up:

[touilleMan]

@shamrin I've just released version 0.6.0 (https://pypi.org/project/triopg/0.6.0/)

[shamrin]

@touilleMan Nice, thank you!