feat: update stdlib mapping for go1.15

- drop stdlib for go1.13
- generate stdlib for go1.15
-  update CI configuration.

stdlib/go1_15_net_textproto.go

@@ -0,0 +1,32 @@
+// Code generated by 'github.com/containous/yaegi/extract net/textproto'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"net/textproto"
+	"reflect"
+)
+
+func init() {
+	Symbols["net/textproto"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"CanonicalMIMEHeaderKey": reflect.ValueOf(textproto.CanonicalMIMEHeaderKey),
+		"Dial":                   reflect.ValueOf(textproto.Dial),
+		"NewConn":                reflect.ValueOf(textproto.NewConn),
+		"NewReader":              reflect.ValueOf(textproto.NewReader),
+		"NewWriter":              reflect.ValueOf(textproto.NewWriter),
+		"TrimBytes":              reflect.ValueOf(textproto.TrimBytes),
+		"TrimString":             reflect.ValueOf(textproto.TrimString),
+
+		// type definitions
+		"Conn":          reflect.ValueOf((*textproto.Conn)(nil)),
+		"Error":         reflect.ValueOf((*textproto.Error)(nil)),
+		"MIMEHeader":    reflect.ValueOf((*textproto.MIMEHeader)(nil)),
+		"Pipeline":      reflect.ValueOf((*textproto.Pipeline)(nil)),
+		"ProtocolError": reflect.ValueOf((*textproto.ProtocolError)(nil)),
+		"Reader":        reflect.ValueOf((*textproto.Reader)(nil)),
+		"Writer":        reflect.ValueOf((*textproto.Writer)(nil)),
+	}
+}