Fix FastAPI version on generated requirements.txt

[Kludex]

Closes #58

[codecov-commenter]

Codecov Report

Merging #61 (1d6de2c) into master (b816c51) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   83.68%   83.68%           
=======================================
  Files           7        7           
  Lines         141      141           
=======================================
  Hits          118      118           
  Misses         23       23           

Impacted Files	Coverage Δ
manage_fastapi/config.py	85.71% <100.00%> (ø)

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