Remove Python 3.6 support

[jodal]

• Require Python 3.7, as 3.6 has reached end-of-life
• Use lazy annotations instead of strings

[codecov]

Codecov Report

Merging #118 (86a0746) into main (2564682) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #118   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          823       835   +12     
  Branches       121       121           
=========================================
+ Hits           823       835   +12     

Impacted Files	Coverage Δ
src/biip/_parser.py	100.00% <100.00%> (ø)
src/biip/gln.py	100.00% <100.00%> (ø)
src/biip/gs1/_application_identifiers.py	100.00% <100.00%> (ø)
src/biip/gs1/_element_strings.py	100.00% <100.00%> (ø)
src/biip/gs1/_messages.py	100.00% <100.00%> (ø)
src/biip/gs1/_prefixes.py	100.00% <100.00%> (ø)
src/biip/gs1/_symbology.py	100.00% <100.00%> (ø)
src/biip/gtin/_enums.py	100.00% <100.00%> (ø)
src/biip/gtin/_gtin.py	100.00% <100.00%> (ø)
src/biip/sscc.py	100.00% <100.00%> (ø)
... and 2 more

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