replace pgtype with pgx/v5/pgtype

[vadimi]

It looks like in pgx v5 github.com/jackc/pgtype package needs to be replaced with github.com/jackc/pgx/v5/pgtype

https://github.com/jackc/pgx/blob/f803c790d0999e9028cbffefad9df02e4aff913a/CHANGELOG.md#merged-packages

as mentioned by @kohenkatz in PR #94

[vadimi]

@georgysavva I also feel like some of the docs in in doc.go don't make much sense in v2 of the package, especially around custom types

[codecov]

Codecov Report

Merging #97 (a5421a1) into master (3003c11) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   82.81%   82.74%   -0.07%     
==========================================
  Files           5        5              
  Lines         512      510       -2     
==========================================
- Hits          424      422       -2     
  Misses         73       73              
  Partials       15       15              

Flag	Coverage Δ
unittests	82.74% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pgxscan/pgxscan.go	70.00% <ø> (-0.74%)	⬇️

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[georgysavva]

@vadimi Thanks for this PR!

@georgysavva I also feel like some of the docs in in doc.go don't make much sense in v2 of the package, especially around custom types

I fixed the pgxscan docs part regarding pgtype. Did I get it right? What else do you find irrelevant for the v2 package?

[vadimi]

@georgysavva this is great, thank you