Replace codecs.open with built-in open for UTF-8 file handling #60

Job	Run time
lint	11s
compatibility (fluent.syntax==0.17.0 six)	12s
compatibility (./fluent.syntax)	9s
compatibility (fluent.syntax==0.18.1 six)	11s
compatibility (fluent.syntax==0.19.0)	10s
test (ubuntu-latest, 3.13)	10s
test (ubuntu-latest, 3.12)	8s
test (ubuntu-latest, 3.14)	9s
test (ubuntu-latest, 3.10)	10s
test (ubuntu-latest, 3.11)	10s
test (windows-latest, 3.13)	24s
test (windows-latest, 3.14)	25s
test (macos-latest, 3.13)	14s
test (macos-latest, 3.12)	18s
test (ubuntu-latest, 3.9)	9s
test (macos-latest, 3.9)	13s
test (macos-latest, 3.10)	26s
test (windows-latest, 3.10)	52s
test (windows-latest, 3.11)	32s
test (windows-latest, 3.9)	25s
test (windows-latest, 3.12)	25s
test (macos-latest, 3.14)	14s
test (macos-latest, 3.11)	14s
	6m 31s

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